Another option for A, if they have superhuman levels of compassion and understanding, would be to say something to C and D along the lines of "yeah, that sucks too. Is there anything I can do to help you with that?"

Sure, but when A is expressing frustration specifically because they're feeling their compassion and understanding breaking down, what then? We're all human, and we all need help and forgiveness sometimes.

The initial framing might also be a target: for example, A could give a specific story about a time they felt their desires and feelings weren't being respected and focus on how much it sucks rather than getting others to change right off the bat. (Hopefully triggering empathetic cooperative behavior rather than zero-sum resource scrambling. Might require superhuman patience and restraint.)

It often does, especially when A is describing processes that lead B, C and D to consider A as socially worthless. That's a big part of the problem - we tend to not even bother thinking about whether A has a point if we don't like A or need something from A to begin with.

Regarding derailing and misery poker being deliberate: I'd be interested to hear what you think of Are Your Enemies Innately Evil?.

I think it's an incredibly accurate assessment of the situation, which just redoubles the tragedy. Even people who deliberately play misery poker and who deliberately derail aren't doing it out of abject evil, they're doing it because they're part of a process that finds their actions advantageous.

I have a hard time calling people 'evil' simply because I tend to not see people as experiencing much agency in their lives.