Stage 2 of Massing of Osgiliath says "Each player cannot play or put into play more than 1 card from his hand each round"

Does this mean that I cannot use Sneak Attack at this quest stage? It seems to me that I cannot because then I would be playing/putting into play more than one card, but I wanted to be sure.

Thank you.

Yes you are correct. If you played sneak attack, you would attempt to put a card into play, but it would stay in your hand since you can only play/put into play one card per turn.

I agree, you cannot meaningfully use Sneak Attack. If you tried playing it then it instructs you to put an ally into play, doing that would then be the second card played/put into play and thus be disallowed by the quest, causing the effect to fizzle.

Thank you very much for the replies! That answer seems very reasonable.