Honestly, I think Marsh would be extremely sensitive to the fact that he is open to control by a shard, even a generally benevolent one like Harmony, after his traumatizing experience with Ruin. He also found out experimentally that he could reduce Ruin's control over him through spikes in his already unusually gifted Determination. I would therefore posit that he's probably taken steps to allow himself to compound Determination.
Not only would that make him extremely difficult to influence, it would probably make him almost irresponsibly certain in the rightness of his actions. Whether he is sane or not is an excellent question, because he's definitely up to something. Could be good or bad or some super position of both.

Recall the mist-sickness and how Preservation forcibly "snapped" a bunch of people who, under normal circumstances, never would have become allomancers. He was able to do this because many Scadrians are born with an affinity for the metallic arts (IE: they have the theoretical ability to manifest access to a specific allomantic and/or feruchemic power) while lacking the necessary Connection/Investiture to be able to ever bridge the gap to gain access without external influence.
Basically:
Metallic arts powers are assigned at birth
Some (and maybe most or even all) Scadrians are born with theoretical access to a power but, for reasons that are not totally clear, they can't snap without additional Connection/Investiture being supplied to them from an external source
Fine details are largely ROF'd at this point; it is generally expected that many details will become clear with the next book

Liquid metals can be burned yes. Wayne demonstrates this trick by melting a lump of bendalloy in hot tea to make swallowing it easier. Bendalloy has a melting point of approximately 70C and tea is generally served at approximately 72C.
This trick is useful because bendalloy burns so quickly that a relatively large amount is required for any kind of sustained use to be possible. (IE: Wayne needs to consume something like 10-100x the volume of metal that Wax does when preparing for a fight).

I've been off the forums for a while, but my general recollection was that the prevailing theory was that "pushing" past "mistpoint" would gradually result in increasingly greater damage to the allomancer's spiritweb unless Sazed decided to relinquish control of Harmony to them (which probably wouldn't happen). This would essentially cap any allomantic expression of power, either human or mechanized, by causing the conduit channeling the power to dematerialize once the threads of its spiritweb were shredded/consumed/separated/vaporized/ect.
In a nutshell: allomancy can't be broken to create infinite or insane power loops because doing so would vaporize the user, be it man or machine. (Caveat: except if Harmony is dropped)
Has any of that been canonized or fallen out of favor?

The powers should absolutely stack, however, this might not be as crazy powerful as could be expected.
For example, duralumin causes the user to very rapidly metabolize the other metals they are burning, much like flaring but on an extreme scale. Flaring is much less efficient than normal burning, IE: the user get more Investiture flowing through them per second when flaring, but the ratio between the Investiture flowing through them and the rate at which they burn metal decreases. Flaring duralumin should obey this principle like any other metal, and having duralumin nicrobursted should be roughly equivalent to super flaring it.
Basically, this is going to lead to cascading waste with every additional iteration of nicrobursting and/or duralumining the end metal. It's mathematically possible that combining multiple augmentations could result in an equal or even weaker final display of power, with the caveat that much more metal would be consumed in the process. Further, even if the end user has many pounds of metal beads swallowed, it is entirely possible that they could have a maximum effective rate of Investiture output that is independent of the amount of augmentations on them; ie: they can be forced to burn pewter faster and faster, but they might be capped at 10x power no matter how fast they burn it.

Why is no one picking Ambition? I don't know what kind of magic system he/she would've come up with, but a planet of Ambition is bound to be loaded with opportunities for advancement and a mass producer of worldhoppers to carry their lord's will to the far reaches of the Cosmere.
Being a worldhopper gives one the best chance of accumulating access to multiple magic systems and frankly I think Ambition would have created the most had it not been for the untimely death...if I'm hedging my bets, that's the way I'd have to go, Harmony just doesnt create enough metallic arts users or I'd go with him.

I think this raises an interesting conceptual solution for what may eventually be a real social problem for Scadrians: the detection and regulation of hemalurgy. If people with abnormally significant Connection to Ruin naturally repulse, or affect by proximity, people with abnormally great Connection to Preservation, that implies to me that people, or constructs like Kandra, with hidden spikes should theoretically be able to be picked out of a crowd if the right technique or technology were utilized.
It sort of points to the potential existence of a natural visceral response to Investiture by Scadrians, particularly in unbalanced or unnatural cases like Inquisitors or Mistborn who enjoy murdering the innocent, which should be able to be manipulated or augmented.
Maybe I'm grasping at straws but that's my two cents. Have an upvote for a solid set of scene analyses.

@Calderis While I don't like to admit it, that makes a certain kind of sense and is consistent with other innate magic systems in the Cosmere like Awakening in which the user needs no special training, preparation, or severe emotional trauma...
I am biased. I want feruchemy to require snapping for symmetry with allomancy and for what I perceive as more elegant alignment with the greater mechanics of hemalurgy and the medallions (which I think we both agree lean on hemalurgy to an extent). I grudgingly agree that your explanation is technically satisfactory with the caveat that, stylistically, I find it less appealing than the alternative. Touche and well played, you're probably right.

I would argue that in the absence of clearly canonized information, feruchemy and allomancy should be assumed to obey parallel spiritual mechanics, after all, they appear to interact with hemalurgy and medallions in equivalent ways.
I posit that snapping must be a fundamental aspect of feruchemy unless explicitly countermanded by a WoB on the basis that both allomancy and hemalurgy necessarily require the cracking of one's spiritweb to facilitate participation in the corresponding magic system. Note however that the way this is achieved differs between allomancy and hemalurgy, the former requiring a calibratable level of physical or emotional trauma and the latter simply requiring a forced foreign injection of Investiture into one's spiritweb.
How does a feruchemist snap? Well, I would speculate that it parallels allomancy in both era 1 and 2. A full feruchemist is, by my reckoning, a roughly equally efficient killing machine compared to a mistborn but on a slightly more limited time-scale. Perhaps they snap slightly more easily then, but, given the abuses and castration of the Terris, it isn't difficult to imagine that they easily maintained a 100% snapping rate under the lord ruler...

Functionally, the aons seem to act like a programming language, with the order they are drawn being analogous to their line number in a script (which generally corresponds to order of execution for simple code). To the best of my knowledge that list is all we have so if you're going for an art project you're going to probably need to go non-canon.
If you want a friendly suggestion: Aon Rao looks to me like it could be drawn with smaller aons inside each circle and used as a comparison statement or mathematical modifier. You might consider picking a handful of Aons to represent the classic logic circuit functions (and, or, not, nand, ect.) And/or mathematical symbols and translate a simple javascript program into them.

I'm inclined to agree with this, and the analogy to hemalurgy seems apt.
Expanding on the specific process flow as I understand it: The spren exists mostly in the Cognitive Realm, the listener pulls it more into the Physical Realm by bonding it to their gem heart, and the bond they establish with each other in those two realms tweaks the listener's spirit web (aka how the listener manifests in the Spiritual Realm) which then prompts a bulk change to how the listener manifests in the Physical Realm.

It just looks like a bunch of mirrored P's to me (d's/b's/q's as well I guess). Shard of Reflection? Shard of Purity (or any other P-word) that has been splintered into 6 pieces? Shard of DQ? Shard of Dairy Queen? Shard of the Blizzard? Oh my God, it's Frost, the Dragon!!!
Wow, 700 posts, who knew?

Let's just list our observations to start:
They had a permanent fortified outpost with a relatively small garrison on the outskirts of Scadrial. They were prepared and actively planning to steal the Shard of Preservation should the opportunity arise but they didn't commit that many resources to doing so, assuming their number of members is meaningfully larger than the half dozen or so people we've seen, which might be a big "if."
Stealing a Shard is a big freaking deal, unless you literally don't care which one you get, and you're just waiting for one to die, which over the last 3 millennia in the Cosmere, is really just a short matter of time for anyone with immortality.
I posit then that the Ire have outposts on the outskirts of most or all shard worlds, fueled by conduits to the Dor, and are attempting to position themselves to steal Shards as Odium and/or Autonomy attack them. They don't care which they get or in what order, so I have to assume they intend to do something that isn't influenced by Intent. The first thing that immediately pops into mind is recombination of the Shards; save for Autonomy, I don't think any of the Intents we have seen preclude recombination if the bearer(s) were so inclined. The second is the destruction of the other Shard bearers; Odium and Autonomy are super dangerous to everyone, and when they run out of peers to murder, it's reasonable to assume they won't just stop and be happy, so a proactive strategy to garner and consolidate resources against them is quite prudent in my opinion, as they are too powerful to face head on for most groups/forces.
Enjoy and discuss!