I just tested and it all works on macOS 12.6 and FCP 10.6.4 and it works just as y’all want it to.
I can make multicam clips from multiple Favorites ranges on clips, select them and make a multcam clip out of only the Fav ranges.
I can create mutliple Favorite ranges on one multicam clip, then Cmd-E and they’re all in the Timeline.