I've never used corn sugar for anything except carbonation. Sometimes I'll use cane sugar in ciders to boost the starting gravity, but in ales, it's hops, malt, water, yeast. The original beer purity laws said hops, malt & water. Yeast was discovered later and once brewers started cultivating it, the law was changed.
Actually had some poser at an ale fest ask me if any of the ales on tap were "Four ingredient ales." I asked him, "What four ingredients did you have in mind?" He didn't know! Poser! Other than the Kona with coffee, they all were.
Yeast is alive and it will just keep munching until there is nothing left to eat OR the ABV is so high it kills them. So, if you put too much sugar/malt in a batch, you are left with a stuck ferment and a sweeter ale than you would want. That's where champagne yeast comes in, it can handle ABV's of 14% without changing the flavor and people will add it to stuck ferments and high gravity ales, like barley wines.