No, Valve's rule is that you can't sell Steam keys at a lower price in different places. You can sell the game at any other price without the potentially-unlimited Steam keys they provide to devs.

Steam key rules and guidelines in the Steamworks docs: https://partner.steamgames.com/doc/features/keys#3

Only for Steam keys

Yes, that's the "collusion". Pretty much every platform has similar rules. I don't think collusion is the right word, as that implies some sort of agreement between them, when in reality they each just have the same agreement. But I do think it's a valid complaint and the implications of such a suit are wide-reaching for that reason.

Yes, there is a rule and contract when selling games on Steam. Microsoft knew this day one, as any other publisher (not sure why Microsoft is singled out here). Valve wants to have price parity of games sold on Steam and other stores. But exceptions are made for short term sales.