Help to find the protocol for JJRC H31 please!

If you only have a 7e you won't be able to use an external module, only the T8SG V2 plus can do that.
(The T8SG V2 plus is just a better 7e )

Yes, a module with an actual LT8920, connected into the module bay of the T8SG V2 plus.

like that but with a LT8920 instead of a NRF24L01:

Because it doesn't have an external module bay, same reason you cannot use a R9 or Crossfire module with the 7e.

No, it doesn't work the same as internal modules.

Yes but not directly connected to the SPI bus in deviation (we don't want to support a 5th internal module).
External or nothing
But just wait anyway ... we might be able to make it work with cc2500, but as I've already said there's no guaranty.