Hmmm... These variants make me think there's a good hybrid somewhere between them that would work better, but I'm not sure what it is.
Shyster 1
I like the VP bonus, and I think tying it to a card that trashes and deals out Curses is good, because it encourages the depletion of piles. I think this is my favorite version.
Shyster 2
I think this version is way too weak for the player. Sea Hag works at $4 with no benefit to the player, but as a curser this compounds that problem by requiring you to discard Silver. I know you have some other cards in the set that trigger on discard, but typically this means -$2 is all the player gets out of it. If you're not going to be cursing, then there's no point to playing Shyster for the +Action unless you're activating Conspirators or something like that. In order to make this a non-terminal Curser, you have to discard $4, which is a lot of coin, especially since a Silver-centric deck will be light on Golds. Also consider that it will be very hard to chain when you have to discard 2 cards each time in order to keep the chain going. Your hand's only so big, and if your deck is mostly Silver, you won't have tons of card-drawing.
I would consider tying the two options together, as in, "You may discard a Silver. If you do, +1 Action and each other player gains a Curse." I'm not sure quite how strong that is, though.
Shyster 3
My first impression is that this card isn't all that great. It's a one-card trasher that attacks your opponents when you can trash your Estates and Coppers. Silvers are a relatively neutral card, but every time you trash one, that's $2 less you have to spend that turn. So I don't see it as brokenly powerful even without the gain effect.
But I don't think I have a very good idea of how it would play, so take that with several grains of salt. It might very well be as strong as you suspect.
In the end, I guess my main thought is by trashing or discarding Silver, you're hurting your current hand, so the benefit needs to be real. When you play Shyster, you'll go from being able to afford a Gold or a $5 Action to settling for a $4 Action or a Silver. The Silvers can keep the Shyster going, but it's not getting you closer to buying a Province. Most Attack cards give a benefit to the player other than the attack on the opponents, usually +$2 or +2 Cards. Trashing cards can be a benefit, but trashing Silvers is usually not. So I like Shyster 1 best with its VP bonus.
I like the flavor of the card. I hope it ends up as a good, balanced card. You can certainly play-test with more than one version before deciding which one to include.