When you choose an internal channel as a source for sidechaining, this is automatically solved by installing an SC-Send in the source channel. Problem: if you solo the destination channel, the source channel will get muted and the external keying will stop. Switching to pre-fader send or solo-safe in the source-channel won't help or leads to unwanted results. My recommendations:
a) The sidechaining source should be pre-mute, post inserts. In an analog environment one would use an insert send on the patchbay for the key signal. If there is really a need for a bus, one could still define one.
b) Please let the user decide, whether the pre-fader sends are pre- or post-mute, if you still let the feature as it is now.