even if the current version seems to have confused you
Let the record show I'm not the person who was confused by current Sewers. (-8 I was just thinking out loud about an interesting alternative.
Would I be allowed to trash Mountebanks Copper with Sewers and Curse with Watchtower instead of both with Watchtower? Does it matter? (Edge case challenge.)
For all it matters, the rules clarifications on the wiki state "A player hit by Mountebank gains the Curse first, and then the Copper.", so they're gained one at a time, so no.
How would same-place-Sewer react to Swindler, Giant, and Knight attacks?
My preferred option, for maximum fun, is that basically works liike Inflation in Fluxx: something that tells you to trash 1 card you can take as meaning 2, trash 2 can mean trash 3, and so on. It's modifying existing card text in the same way Lantern does, rather than being a tacked-on extra. Things which apply to the trashed card apply severally to each.
So...
Swindler: You may instead choose to trash the top two cards of your deck (blind, and both at once). If you do, your attacker chooses two cards for you to gain (then you gain them in an order you choose).
Giant: There's only one revealed-from-top-of-deck card, so you can't choose to trash a second.
Knights: If both revealed cards cost $3 to $6, you may trash both. "If a Knight is trashed by this" is triggered by either or both trashed cards being a Knight.
(This should probably already have moved to the variants forum. Sorry. I'll migrate there if there are any more questions.)