Minstrel
Actually, I meant for the card to work the way you wanted to suggest. "They" in my wording meant "the other players, as a group." If they discard any Treasure cards, you gain one and only one coin. Maybe a better wording would be "If any Treasure cards are discarded, +$1. If any Victory cards are discarded, +$2." Or, "If at least one Treasure card is discarded..."
So in a 2-player game you get $0-$2, and in a 4-player game, it's $0-$3. $0 is still more likely in a 2P game though. And it does have that weird situation of, well he already discarded Victory, so there's no reason I shouldn't. It's not always a balanced attack. I don't particularly mind that, though, but I can see why it might not be ideal.
The original card idea DID have +$ for each card discarded, but I realized that wouldn't scale well. Still, it might be a little weak as is. With good trashing, you can clear out your Treasures and Estates and rely on +$ Actions. Then Minstrel is almost useless until the endgame. Discarding 2 cards for +$2 is a significantly weaker Vault-like benefit, and really works best with a bloated deck.
Mage
I'm glad to see you like it! Another reason the Copper applies to the second half is that the -1 Action is not optional. So playing it when you have more than 1 Action left will burn 2 Actions. The Copper part guarantees you still get something for that after the Curses are out, so long as Coppers remain.
If I do change the bonus to something weaker, then I think I'd need to add an "If not" benefit for if the -1 Action fails. Otherwise, in a game without + Actions, the card would be about equal to Witch but $2 more expensive. I guess I'll see how it plays and go from there.
Gemstone
Yes, I imagine this card will be rather swingy. It very much encourages everyone to aim to get up to $9, or $8 at least, as quickly as possible. Building up an engine that takes time to get going might leave you in the dust. On the other hand, in order to play it, you'll want more than one. I wonder what the optimum number to buy is. Is it better to have 1 $9 Gemstone or 3 $7 Gemstones? Especially since each one you buy makes it easier for your opponents to buy in, which both helps their deck
and hurts yours. I could see an eventual change basing things on how many cards are left divided by 2, giving it a narrower range of costs and benefits and slowing down the change. But that's only if this doesn't work out.