THEMA:

In the CB200 you configured Rx2 as Exbus, but you connected the Rsat2 to CB200 via ppm?

And why do you use 8ch ppm only, the diagram says 15ch ppm?
What is the failsave setting of the assist receiver?

I would try to connect both the Assist and the RSA2 to the CB200, both on ex-bus protocol,using receiver 1 and receiver 2 connections. Important to set CB200 to be ex-bus input and the Assist and RSA2 as ex-bus output.

I don't like that use of double chaining the receivers, and I like even less the fact that you are using both ex-bus and ppm as protocols to talk to the CB200.

One mention about failsafe. I'm not sure where you are programming failsafe, but my practice is to set it on the CB200 and turn it off all together on the receivers.