Is Douglas really helping out more late than Zealot is early and mid game? If you used Zealot up until the point Douglas joined (You've likely got several unpromoted characters up until Chapter 16, so he'd be at least tolerable the whole way until then), he's not even really much worse for awhile. Lots more Speed and Move, sword use, possible support (If either Treck or Fir is not played, Zealot is Noah's only option for a B support. Treck gets a B or A with Noah depending on whether Noah is supporting Fir, and his non-Zealot options for a second support take forever.) Much less Strength and durability, obviously (though the Speed difference is such that if Zealot doesn't double the enemy, Douglas may well be getting doubled), and probably losing overall, but not getting completely shut out IMO, which is telling seeing as how Zealot was actually decent for awhile before this and best on team for awhile before that.
Cath > Astol seems pretty crazy to me, but the gap between them is probably much less than the gap between a High Tier combat unit and a Bottom Tier combat unit. I'd certainly move her at least into Low.
The gap between Ellen and Saul seems questionable, the one between Elphin and Lalum even morso, and Lugh might be more deserving of Upper-Middle considering his rather scrubby beginnings.