Re: GM's butting into PM's
It's technically correct, but also probably an oversight. What's happening is that you are changing the participants from (GM NPC, PC, PC) to (GM, PC, PC). The GM NPC character is no longer within the list of recipients, so the list of recipients has changed, and thus it splits off into its own thread.
When a GM in other normal circumstances inserts herself into a private thread, though, it's going from (PC, PC) to (GM, PC, PC), but the primary list of recipients hasn't changed. That's why the two situations are different.