1. As written, "Discard all..." seems to represent a one-time action that happens when you play a card (although the rules aren't particularly clear here). That said, I think
Many-tentacled Creature was supposed to mirror the original
Keeper of Westgate, so I would play it as a "while" thing.
Given the above, you won't get twilight from
So Polite because Gollum is discarded when he comes into play. The rules mention "effects triggered" such as "When you play" / "Each time you play," but considering that the
Northern Ithilien cheese was stopped by this rule, I think that extends to the cost/effect scenario as well.
2. Don't have the details here, but it seems Decipher ruled that the
Corsair Marauder would still get his effect. The rules to me don't seem to support this (and this topic was discussed a few times), so again personally, I'd play it so that the Marauder won't get his effect.