I see it through JM lens where you can't reliably renegotiate the CJ due to ephemeral coordination so I've always just ignored RBF there, (and nor can you stop someone retrospectively cancelling the coinjoin contract by double spending their input). Of course CPFP exists but since there's no penalty mechanism I don't think the whole pinning thing is so relevant?

Maybe with adversarial makers in a taker maker model? It seems unrealistic.

But with redesigned cj protocol maybe other issues arise.

To be clear I really don't know.