What are some pros/cons of restarting a bread machine mid-cycle?
I'm not familiar with how bread machines even work, so I'm not sure if you mean that you are restarting the cycle and that means that (A) you are just mixing the dough longer, or (B) the dough mixes, then rests, then mixes, then rests, etc. Either way, not a good idea.

For scenario (A), you're overworking your gluten. If you do this, it's just as bad as undermixing. You can find plenty of information on the perils of overmixing.

If it's (B), it's also not a good idea. When bread rests, you should never mechanically mix it again. You only need to 'punch it back' to expel the carbon dioxide. Mechanically mixing at this point will actually break your gluten threads. Also, adding extra yeast or sugar is just a bad idea in general. You are not going to increase any flavour, certainly not going to create a sour. For that, you need to learn how to create, maintain, and use sours / sponges.

I am not speaking flippantly here, but I would suggest reading a proper baking textbook to learn about the theory of breadmaking to enhance your knowledge and understanding of ingredients and their role... 

Having said that, I'm sure someone will come along and refute everything I've said - happens here all of the time. I only speak from 20+ years of experience plus baking and pastry papers.