First, I'm not absolutely sure of the nomenclature here. On FCU I'm trying to install the takedown safety lever but it appears that the sear is rotated down and keeps the safety lever from seating properly. And the sear pin does not budge (broken thumb nail to prove it). Any help/suggestions are appreciated.

The takedown lever should rotate down and when doing so pull the sear down with it so the slide clears it to install fully.

The strut connected to the takedown lever should have a finger engaging the sear to pull it down when fully rotated.

The 365 will joggle parts loose when removed from the grip unit, unlike the 320 which was designed to prevent it.