One last note: The solution I decided on for "overleveled" demons in a ghost's party was to simply... de-level them. Demons in a Relic Wraith's party that exceed a certain small threshold above the ghost's actual level will be de-leveled down to that threshold. i.e. A Level 3 summoner would have any demons over Level 4 or 5 so leveled down to 4 or 5.
Note that the usual "distance from starting level" XP penalty demons receive only applies from their "in the wild" starting level. So if you recruit a Level 4 Raiju from a wraith, it still has a starting level of 16 for the purposes of that penalty, and thus will level at full speed until 16, before slowly beginning to slow down like any other Raiju would. Note also: the penalty does not work in reverse, so it doesn't level faster to try and catch up to 16.
Thanks to gaswafers for suggesting this (if for a different problem) in the overleveled allied demon thread. Might still do it for that too at some point, but not ready to pull the trigger on that yet. (I'm much more willing to do it on Relic Wraiths because I'd rather these err on the side of caution initially.)