Can I make invert syrup in situ when making candy? There are a couple recipes out there for invert syrup that basically say add some acid (citric acid or cream of tartar) at 0.1% w/w bring it up to 240&nbsp;°F​/​116&nbsp;°C then let cool. However, is there a way I can be lazy when making candy and in one pot invert about a quarter of the sugar to avoid having to make and store invert syrup and worry about if I have any?

The fact that many of the invert syrup recipes also say to not go past whatever temperature, is a bit curious, will it begin to caramelize at a lower temperature (240&nbsp;°F vs. 300&nbsp;°C) or what's the issue?  I'm not sure if I could just add 0.025% w/w tartaric acid to straight table sugar and call it good when I go up to 280 °F or 300 °F for soft/hard crack.
Yes, you can do it. In fact, most recipes for candy I have followed suggest to do it that way. Just add your acid before starting to heat the syrup, and proceed with the candymaking as usual. 

I suspect that the reason the recipes for inverted syrup specify an upper limit for the temperature is because they are recipes for syrup. If you overheat them, you will get, as you noted, soft or hard crack, but not syrup. If this is what you need, you can do it, it is just not the intention of the recipe author, so they warn against it.