Here's a game between Adam Horton and Qvist which showcases several features of Masquerade very well:
https://www.youtube.com/watch?v=JkogD_WLqG8&index=17&list=PL7MIC-UQuoeSnDnNa2HoWR85tod7J7Tqj. I recommend watching it, to see how some of your concerns play out in practice—or at least one way they
can play out.
The constant threat of being forced to pass your opponent a good card in exchange for junk just screws over so many other aspects of the game. It feels like Possession in that you have to keep your deck crummy so that your opponent can't take advantage of it.
They're not quite comparable; an awesome deck with 20% bad cards passes bad cards to Masquerade but gives good turns to Possession.
Also, as you see in the linked game, even when forced to pass a good card, it looks like it's not a huge deal. Maybe Qvist could have played some more Monuments if he had passed fewer Mining Villages, and maybe Adam could have bought more green cards if he had passed less Silver, but on the whole it looks to me like it was much closer to having a mild impact on the game than causing huge swings.
-Should you not even bother with other trashers when it's on the board, simply because trashing down too fast will just give good cards to your opponent?
Trashing down fast with Chapel seemed to work out perfectly fine for Qvist in that game. Keeping a few bad cards around to pass to Masquerade makes sense when trashing down fast. Obviously the larger your deck, the more bad cards you need to have a high probability of having one in your opening hand.
-Are there any times it can be ignored? The only situation I can think of is Ambassador, which is a great counter because it's also a card that can thin and junk at the same time. Are there any other situations?
I would think any engine that draws deck and has some vaguely decent trashing makes mid-game Masquerade a complete non-threat. Upgrade, Forager, Junk Dealer all do a fine job. Steward and Chapel both do the job very well.
Given the choice I probably want Masquerade over Forager in the early game, due to the +2 cards, but I pick Chapel over Masquerade any day. I'm not sure between Steward and Masquerade: Steward trashes faster but also gimps your turns. (Chapel of course totally gimps your turn, but it gimps a smaller number of turns, and gives you really big turns faster.)
Note also a reason to
not ignore Masquerade: when it's the only draw. I think Qvist's deck would've rather used Moat for draw if available, though. The presence of +buy/gain might swing a decision between Masquerade and Witch for draw—which is a choice you almost never have to make, so whatever :-)
-What about other junking attacks? Should you not bother with them because your opponent can just fling the junk back at you? It seems to me that Mountebank and Cultist are fast enough to bury an opponent who's relying solely on Masquerade, but the other junkers I'm not sure.
I wouldn't worry about getting junk returned. I think the real concern is whether the trashing speed of Masquerade makes the junking worthwhile or not. When the junking is faster than the trashing it seems great to go for. When it's equal, meh, I don't think I can give generalized advice. One general true statement, though: when trashing the junk you're given, it's (often) at the expense of trashing the junk you're starting with. Delaying your opponent's thinning may be worth it.
-Is buying more than one of them ever a good idea?
Qvist does it in the linked game, and it seems perfectly sensible to me; though he does it for the draw.
If playing Masquerade/BM, getting 2xMasquerade makes sense to me. The earliest you'll get rid of your starting cards is turn 12, Smithy/BM gets 4 provinces by turn ~14, and
http://forum.dominionstrategy.com/index.php?topic=2532.0 ranks Masquerade/BM as stronger than Smithy/BM.
-Is passing good cards to your opponent just not that big a deal sometimes? Can you trash all the way down and build an engine so good that it can shrug off all of Masquerade's mischief?
I think Qvist succeeds in doing just that. The effect of Adam's Masquerade is not nothing, but it's also not a big deal.
(DG says "One player might want only actions cards and the other only treasures."; the linked game is an example of this.)
Any help on dealing with this insane card would be much appreciated. Thanks.
I think you overestimate how strong the attack part of Masquerade is. Passing a good card feels frustrating, but the intensity of the feeling and the impact on the game are not always proportional. Here's a random suggestion: on the next kingdom where Masquerade shows up, get one or two, but don't go out of your way to play around the risk of getting junk or having to pass good cards. After the game, estimate how much of the game outcome is due to Masquerade-junking or good passes.