The new data being mixed in is a new ephemeral key that is generated as the root ratchet is turned, which in turn derives the chain keys. That new key would only be visible/accessible to an attacker if the users device was completely compromised to an attacker. At which point nothing is going to be useful.
Discussion
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.