Welcome aboard
@ewhee
1/ Could be a booster or perhaps the brake distribution block (LHF chassis rail - access from underneath, near the LH lower control arm).
2/ Vacuum leak is my guess. Check all hoses and replace anything cracked/broken/manky looking.
3/ Depending on which one it is, you might be able to drill it out and re-tap it easily enough. Otherwise, a decent exhaust shop may be able to help and save you a lot of swearing.
4/ Dont be swayed to run urethane everywhere for bushes. If you're after something to tighten the car up and make it a little more sure-footed feeling, there are some good places to use urethane while there are others where rubber is a better idea.
This forum is a great source of knowledge, so keep asking questions!
Pop up some pics so we can all see what it looks like! :)