Can I make invert syrup in situ when making candy?
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.