can boiled cane sugar sit covered overnight and used the next day?

Probably. But it doesn't take much time to reboil and proceed to bottling. I would for peace of mind.

 

use a microwave, it takes like 2min max to boil up a sugar solution

 

I do it all the time. I just boil it with the lid on the pot so the steam sanitizes the lid. Haven't had an issue yet.

 

Oh yeah. I would reboil for sanitation, but I have used simple syrup for margaritas that has been in the fridge for 2 weeks without reboiling with no health effects.

 

I doubt that it needs reboiled. I work in a lab and we regularly make growth media, sanitize it with foil over the top, and let it sit for up to a week at a time. As long as you sanitized everything correctly it will be fine. That being said it takes 2 minutes to boil it again so what would be the harm of doing it again anyways?

 

I would reboil. My wife makes simple syrup to have on hand for mix drinks, and occasionally if left in the fridge too long it will grow mold. Better safe than sorry.