The only way I could see the communists being allowed power is with a set of harsh restrictions on what they're allowed to do...

Which when I think about it, the Sejm already pretty much has? Since they can't effect the Crown Council, and they can't put anything through without crown and council permission, they can change voting rights for the Sejm but nothing else.

I think there's really two options that make sense here, and neither of them involve just saying the communists don't have the Sejm.

The first is two use the historically not enforced restrictions on the Sejm to allow them to "take power" but effectively have full veto on anything they want to do from the crown and council, and the second is to abolish it entirely as a failed experiment and change the government back to being a monarchy. The second one is probably practically insane, but we've seen monarchs losing their grip do crazier things to maintain it, and the High Queen is potentially losing faith in the public.