Contract/Bookkeeper - I made a fan card way back that had the "play a treasure, draw to X" mechanic, and I liked it a lot, so I like this card too. I think the part I don't like about this, is that there are only 5 copies of Contract, and sometimes winning that split may be really important. And because it gives +Buy, sometimes you lose the split just because your opponent happened to draw a Contract on the right turn.
Impostor - It looks fine, although a bit transparent that this your attempt at Leprechaun without hexes. The +Buy is a great addition here, I love that part.
Institute - I'd lean towards "broken", even in absence of TFB. If you compare to Distant Lands, this is worth 1 less VP, but you don't have to spend time to get it out of your deck. Also, if they deplete, that's two piles gone. A nerf I would try is, make it a fugitive instead of a lab. But I think it needs a bigger nerf than that.
Convert - I think this is on the weak side, because you can't trash estates, which is usually the most important remodel target. Would it be crazy at $4?
Promenade - This reins in some of the problems with King's court (i.e. crazy with certain actions, swingy because hitting $7 is hard), but fundamentally I just don't like King's Court, so I'm not really a fan.