Can several identical cards be attached to a single enemy?

Sure, as long as it isn't limited by the card's text in some way. Which card did you have in mind?

I can think of limits like that, but for cards that attach to locations: Obscuring Fog and, in a more roundabout way, Locked Door. If the card's text doesn't specify otherwise, multiple identical cards can be attached to the same card.

Towering Beasts in Undimensioned and Unseen.

They can stack, then, because they don't say they can't.

I've had a Brood with 4 Towering Beasts and 2 Altered Beasts, so yes, go ahead.

Jim Scryed into 3 Towering Beasts with one Brood on the board and just said, Screw that guy! That scenario is really ruff if you can get the broods into certain locations to stack clues.