I agree that the source of the ability can't be used. In addition to everything you said, you could play a card in the trash, it's not even your card.
When we're ordering those two things, well we are ordering them by the trigger; the effect doesn't provide a way to order them.
Yes, that's how I've been thinking about it to, based on the Noble Brigand / Embargo ruling - quoted above from another thread. But you don't seem to be following this principle when you conclude that Road Network is ordered by Bob.
when you = it's ordered by youThen it should be:
when another player = it's ordered by another playerAgain, to me this means Alice is ordering Road Network when she buys a Victory card.
Anyway I think what makes sense here is to go by the player told to make something happen.
Are you here changing your mind about going by the trigger?
Ok, let's look at "who's being told to do something" (the actual effect) rather than "who's causing it to happen" (the trigger). Again, this is NOT how it should work based on the previous ruling that "when you" by itself means it's happening to you:
Ill-Gotten Gains / Watchtower: Alice buys IGG. IGG only tells Bob to do something, so in this case Alice has to use Watchtower first, then Bob gains a Curse.
Embargo / Noble Brigand: Alice buys NB. NB tells both Alice and Bob to do something, so then it's unclear who it's happening to. (Of course when we resolve it, it's Alice first, then Bob, but the question is ordering NB vs Embargo.)
Swamp Hag / Haggler: Alice buys a card. Both SW and Haggler tell Alice to do something, great.
Road Network / Haunted Castle: Alice buys HC. Road Network happens to Bob. But HC tells both Alice and Bob to do something, like Noble Brigand.
The way this could work if we want Noble Brigand and Haunted Castle to be ordered by Alice, is that since Alice is involved (she gets Treasures from NB, she gets Gold from HC), it's happening to her. But it also happens to Bob! NB happens first to Bob, then to Alice. HC happens first to Alice, then to Bob. Well... It's Alice's turn, but what if Alice gained HC on Bob's turn? So... it's Alice who triggered it in any case. We're back to looking at the trigger. So then the rule has to be something like,
"if an ability happens to several players, it's ordered by the player who triggered it, otherwise it's ordered by the player it happens to". EDIT: I intended it differently than i phrased it. It's more like: "If an ability (also) happens to the player who triggered it, it's ordered by that player, otherwise it's ordered by the player it happens to."
Then I guess we don't need the additional rule about optional abilities, since those always only happen to the "correct" player. Well, unless there's a Reaction like, "When another player plays an Attack, you may discard this. If you do, each other player gains a Curse". Then we need that rule too, otherwise the Attacking player orders the Reaction, an impossibility.
EDIT: This new rule suggestion breaks down with more than 2 players though, like in your example of "When you buy a Silver, each other player gains a Curse" and the same for Copper. Since Alice is not involved at all, only Bob and Cathy, who chooses which one to do first?
