How does the choice of vegetable ratio and processing steps influence the taste of mirepoix? There are sources on the internet, but each one touches only one or two areas and some contradict each other. For example they say "two parts onions for other veggies" but videos always show equal parts onion to carrot and celery ratio.

I believe that simply explaining the process behind making a flavor base (how the veggies interact with each other, the role of fat, how dicing size affects the end result etc) will help us all make better informed decisions for our own preferences in whatever flavor base we choose to make.
A bit late to the party but ....

Onion and celery have similar effect in the mirepoix, the leeks will have a similar effect but with more depth on the sweet side &amp; carrot will bring some sweetness.

Based on that, depending on what you want to cook, there are some guidelines. 


For stews, keep in mind that some of the next ingredients might add some sweetness to the meal, so it's ok to equal the onion and celery parts to the other veggies. 
For broth you might want to reduce the onion and celery part to 1/2 part of other vegetables (carrot, mushrooms, tomato paste, etc.) knowing it will eventually reduce into a demi-glace and that you will season it with spices and aromates. Reducing mean the flavor will concentrate. 
For soup / potage you should adjust to the main ingredient to balance it out, usually I go with onion and celery in equal part and no carrots unless you want the soup / potage to have a sweet taste, which is ok for pea soup, tomato, carrot, etc. 


Hope it helps,