If an opponent has more cards in hand than their maximum hand size, they'll discard during their cleanup step, which is after the end step
Yes, cards from this expansion are legal in the Modern format, as well as in Legacy, Vintage, and Commander
Sometimes the card you really want is staring at you in the graveyard, and a Regrowth or even Seasons Past would go a long way
Tacking an extract combat step on a reasonably large body is what Karlach is all about
There are a few others out there, but these ones are among the most reliable
It's a classic and well-loved mill card, but its focus on just a single target does make it feel puny in a multi-player MTG format like Commander