But that means you are changing how "+1 VP" is described in the Prosperity rulebook. I mean, someone could have Alchemy and Prosperity, but not Empires, and play with someone who has Empires (or in a tournament where the errata are known), and if they play with Monument and Possession that person needs to be told both about the Possession errata and about the change of how "+1 VP" works (i.e. not put on your mat).
That would not actually play out that way. No-one needs to be told "it doesn't go to your mat, you are just putting it there as a handy place to store VP tokens" because no-one is staring at the rulebook trying to find problems. The one person says "oh hey Possession has errata, you get tokens now" and the other person says "oh including VP tokens" and the first person says "yes."
Right, and then if they play with Pirate Ship, "no", and if they play with tokens from Adventures, "mostly no, but yes on this one".
I don't understand how getting +$1 don't just trigger both tokens? "When you get $" happens for both tokens, and then you resolve them in an order you choose. That's how abilities normally work.
The token is a token; it didn't have space for precise rules text and doesn't have it. It doesn't say "would" but that's how it works; it's not "you get $ and then lose $1," it's "when you would get $, you get less." They apply one at a time and the second one can't apply because you are no longer getting $.
It's very understandable that it doesn't say "would", because it would be pointless (and maybe confusing) except for this one interaction that you never thought about.
The reason you're ruling that you get the other player's -$1 token, is not that it's intuitive or that most people would think so. Surely the exact opposite is true. It's because that's what the abilities happen to technically say. So you're ruling on a technicality which is against how most people would think it works, and it's not even mentioned in a rulebook.
So I guess we're not really talking about what casual players would do, because they would either just figure that the Adventures rulebook is correct and you could never take your opponent's token, or look it up online and maybe find this ruling.
We're talking about the rules and rulings that people who care about them could find online (hopefully collected somewhere, like the wiki here or my rules document). And they should hopefully make sense and be consistent, and not just for instance be a list of tokens that work or don't work with Possession with no explanation why. It's in that context that Pirate Ship works logically so that you don't get tokens from Possession, and you don't get your opponent's +1 Card token. And it's in that context I'm saying that based on what things literally say, you are changing "+1 VP" from Prosperity, and "when you get $" on the -$1 token.
I guess I just don't understand why some things can be fixed, but not others. It seems a bit arbitrary.
Possession says you get the tokens the other player would get. Adventures says you never get your opponents' tokens; nowhere does it say that about any other tokens (obviously because there are no other tokens specifically for each player.) Unless Possession specifically said that you get your opponent's Adventures tokens, I would never think that it overrode the rules for the tokens, given no other information.
Sorry for being a pain in the ass, as is often the case, but I care about the game and the rules, and this just seems to make everything more fuzzy and introduces several more special scenarios that need rulings/explanations (having two or more tokens, losing your opponent's token etc) just for sticking to the literal reading of this one thing when it seems equally legitimate to say the token rules override Possession, which would also jibe with the intuitive understanding.