After completing a room, we got to look at 9 shop item cards and keep 3 (max 1 weapon). Are these supposed to be from act 1, or act1 + 2 shuffled together? I assumed the latter.

Act 1. All act 1 components only until the game actually tells you to switch to act 2. (After the first Boss, Stage 3)

"Heroes also reveal a specified number of Shop Item cards from the current act..."

"After heroes resolve the third stage, they follow a few steps to transition

into Act II..."

You start with Act I cards and then transition to Act II starting with the fourth stage. You might want to re-read the Delve section of the newly updated Road to Legend rulebook on the Decent product page.

Ah the games does not link to new PDF. Yet that's been bugging me