Banker
I really like the on play effect. It's usually (but not strictly) better than Mine, but that card needs a bit of a boost anyway. Not sure why it needs to be limited to trashing Treasures - giving up an Action for a Gold (usually) would be an interesting choice. The reaction is good for tiebreaking while being fairly benign, and it's a meaningful choice between that and the play effect in the endgame.
Duelling Witch
The reaction is a decent idea and I can imagine it being a fun "not so fast!" when supply piles empty. I also like how you tried to make the two halves work well together.
Generally in games with cursers, there's already enough pressure for everyone to buy a curser, and this card just exaggerates it. Attacks that are also moats can also end up being a bit too swingy.
Additionally "top of your discard pile" isn't really a thing in Dominion - if it's going to be on a card it should really justify keeping a tightly ordered discard pile in a game where that largely doesn't matter, and I think the new errata make it matter even less. Gaining the curse to hand simplifies things a little bit.
+1 Buy also doesn't seem like enough of an on play benefit to me to get around looking like Sea Hag (for what it's worth, Skulk is on my Dominion Online banlist).
Mirage Island
This is a very interesting card. I like the way it can turn bad hands into good hands which really mitigates the luck based aspects of the card. Even without other Victory cards in hand it's a good play on the Distant Lands concept.
Persecutor
This will lead to very quick games, which could be dominated by early game luck. I feel like there'd be ways to make this concept more interesting (eg you not being able to buy the cards you put your Exiled token on, or other limitations).
Repurpose
May need some wording changes to prevent the lose track/don't move rule from preventing other players from gaining the card they just bought. If that's what you intended, I don't really like the idea of you not getting what you paid for.
In general however I like how this enables desperate endgame plays. My first thought was that it was a bit weak for $6 but it can be used like Crop Rotation on your estates, trash curses/ruins and as an endgame tool so maybe it's fine there. I think it would be cleaner if it triggered on opponents gaining Victory cards to fix the wording challenges and not trigger quite as often (which can be a bit annoying online).
Jetty
A weaker Wharf is still a very strong card and would likely be well bought even without the Fleet effect, which makes the card less interesting for me. When it changes the outcome, I can imagine the mood being more "you happened to have a Jetty in play at the time" than "Well played, buying Jetties and managing them". There may be a top that could make the bottom work, perhaps one involving deck control, but not the card as is.
Royal Demesne
The card is strong enough that you'd want to build an engine with these anyway (or maybe even BM), and then you can end up getting a lot of bonus points around the endgame without much strategy involved. Not a bad choice for an effect, although I would change the threshold so that there'd be ways to get around the Demesne. This card could be good as part of a split pile or with a weaker on play or higher cost.
Shaman/Totem
Shaman does nothing for you until you get to the Totems, so there's quite a risk there (and counter-play may be to ignore the pile and get the person aiming for totems wasting time). The "reverse distant lands" effect is strong enough that the player buying Shamans doesn't have an overwhelming advantage once the Totem pile gets revealed; all players would be going for them.
Bargainer
Nice and simple. The fact it can trigger on any players turn is nice. It encourages players to win with smaller cards, but keeps provinces in the picture, which I think is cool. Sometimes I think it's a bit strong for $4 but the fact it doesn't help you gain a more powerful (a bit like Devil's Workshop).
Young Artisan
A nice simple $2. Unfortunately the games where it would shine in the endgame are also the games where "don't buy a province" counterplay could neuter it. Still it looks like a lot of fun. I wonder if it would even be ok gaining $5 cards, since it won't activate too many times per game.
Blockade
I can see this one being a lot of fun. Ambassador shenanigans don't bother me much. This will work especially well in games with potential for 3 pile endings - and it will teach players to plan for them or even strategise from the beginning about bringing them on instead of them just being something that "happens" as the game progresses. Unfortunately I don't think it fits the brief particularly well as it doesn't give players strategy around the endgame. Still it's a fantastic idea that I'd love to play with.
Countryside
Similar to Mirage Island, but a bit more straightforward. A Green card that you actually want in your hand as the game is wrapping up. I think you'd generally want this over Duchy unless you're reasonably sure it's your last shuffle, which is similar ground covered by Distant Lands.
Abomination
Cards with the type Curse really do need some clarification around what happens in games with Cursers, especially when they're very cheap cards. It isn't an easy decision to decide when to get rid of this with the +buy so you're on to something. I think this concept would be served better by a higher stakes card costing more.
Royal Pardon
I had to think about this one for quite a bit. A powerful "oneshot" that can resurrect itself and keep the game going. Not sure it needs to be both a Reserve and a Reaction. Tracking may be an issue with naming cards (but it's unlikely to be a frequent occurrence). It really could be a simpler card. Would be interested in playtesting.
Armorsmith / Overcrowded Village
Didn't see the 2 entries and I'd written up Armorsmith so you get a double write up.
The main effect of Armorsmith, which is actually quite strong and useful on play, would be to discourage Province buying. You'd almost always take the Armorsmith (even in BM games). There'd be some interesting strategy around it (since Armorsmith itself would be a reasonably good buy), but not for the player gaining the Armorsmiths. The Armorsmith pile basically modifies the Province pile. Actually quite interesting.
Overcrowded Village on the other hand is not a great card on play. You would prefer almost any other source of +Actions. This card instead encourages players to get in earlier on the Provinces to discourage other players from taking the bonus VP. Buy early provinces instead of engine building - unless Overcrowded Village is the only source of +Actions, then buying the Province helps other players build their engine. Again, interesting.
Both cards create interesting game dynamics, but they don't really make other players active participants in the endgame so much as modify the way provinces are valued in that game.
Con Man
Cards that unconditionally give out Copper have issues well discussed on this forum, but this gets around them by letting you put Coppers back (including on your own turns). It's a bit too much of an auto-buy in terms of being able to junk other players and thin your own deck (like Ambassador), but there are some situations that are interesting with the reaction (eg one player returning their estates when another player is greening).
Eavesdropper
I like it more than Smugglers. It still encourages players to use less powerful cards to get Provinces or pursue different strategies, which is interesting, and has less luck and is easier to track.
I think a better way to nerf the card would be to allow the player gaining the first copy of the card earlier access to it (optional top decking or gaining to hand). Setting up a bunch of these that would trigger a game winning Duchy run (if your opponent buys a Duchy) is actually quite an interesting situation to me.
Brothel/Lust
I think it's just a bit too strong all around (really not sure why it needs a "salt the earth" effect), but I like the idea of a persistent negative effect that sits there until you do something, and "buy a victory card" is an interesting one strategically. I think it would be worth playtesting a bunch of different effects for the state and have the card itself be simple.
Landlord
This card just seems like one nerf after another. I think more about what this card can't do than what it can. At the very least the estate gain should be optional.
Colossus
Too good for other players I think. In games without trashing you'd never buy this, and even in games with trashing I would want to hang on to my Estates so they could continuously generate VP. In games with cursing, I can see players just choosing the curser over the Colossus. Just making it apply to Action cards may be the most interesting.
Stowaway It can be very strong like City in situations where City is strong, but largely I see it functioning like a Duchy you want early game. The more the game progresses (and the more you want an actual Duchy), the stronger the on play effect is. However you won't get to claim the Duchies if they aren't in the supply - which makes Duchy Dancing all the more fun. It might be a bit slow and I think I preferred the stronger version, but it's a great design.
Winner - Mirage Island by Aquila
Runner Up - Stowaway by anordinaryman