Post-compromise security refers to recovery from a total leakage of internal state.

The DH ratchet uses continuous key exchanges to update itself in a non deterministic way.

In your case, this is not actually a DH ratchet, but a symmetric ratchet without any data mixed in (the root) being used to feed the sender and receiver ratchets. This makes the system state determinstic and gets rid of post-compromise guarantees, and has equivalent security guarantees as using a single ratchet for send and one for receive.

I also do not see how the “active participant changing” would be defined in contexts where there’s concurrent events being sent.