Well, I almost never have more than 1 Good companion, so that's that. Abundant on a Paladin is very good as Paladins have spells only up to the 4th circle - which means 1 Abundant covers 3 out of 4 available spell circles. Enduring.. yes, it is too expensive IMHO.
Also, why don't you mention Grave Singer in your weapon list? It is an 18-20 critical range great axe (though it can be made into other axes). I think it's ideal for Seelah on a horse.
Horse is not great for Seelah at all since she starts with two Feats that are wasted Mounted. Now that Magic Weapon, Greater no longer stacks Divine Weapon Bond is too good to give up. She'll be bypassing DR a Chapter earlier than other companions, can add Keen at will, and eventually gets Brilliant Energy which is outstanding in this setting against the bosses where it's needed. Grave Singer can be made either off-hand, main-hand, or two-hand. Not a great fit for Seelah due her having higher saves already so she doesn't need the condition removal nearly as often. I did mention it for MC or another companion. Scimitar is good for Bless Weapon crit-fishing but I also like Balanced Defender when Shield Bashing since it gives +3 untyped AB to both hands.
Mounted Pal with two-hander can be a decent MC if you don't want to bring Seelah. She sets up well for either tanking with Tower or Shield Bashing (doubles up Smite bonus and there are some ridiculous bashing shields) off-tank. Casting MC Pal would require DLC with all Good Companions to unlock Bestow Grace and Holy Whisper (or even Challenge Evil if you can pull off Azata Pal) but I can't get DCs high enough in main game with Seelah to make it work/worth it above Core (not enough Good companions either). Something like Mounted Divine Hunter with Persistent Holy Whisper into Arrows of Law could be good, and your Archon's would be better too.
Key Pal spells:
Lvl1: Veils, Bless Weapon, Divine Favor
Lvl2: Effortless Armor, Aura of Greater Courage, Bestow Grace
Lvl3: Archons
Lvl4: Aspect Greater, Holy Sword, Eaglesoul, Inspiring Recovery
Pal spells to save slots on other casters (letting them focus on offensive casting/non-situational stuff)
Lvl1: Unbreakable Heart, Resto, Lesser
Lvl2: Prot Communal, Remove Paralysis, Stat buffs (not great since lower CL)
Lvl3: Delay Communal, Remove Blindness, Resist Communal (extra casts are good situationally, but lower CL hurts)
Lvl4: Edge, Death Ward, Burst, Resto
Caster Pal (MC and/or Core and below)
Lvl1: Challenge Evil (Enc), Stunning Barrier (Abj)
Lvl2: Arrow of Law (Evo), Litanies (not great since need Swifts for Smite/Mark)
Lvl3: Archon's (Evo), Whisper (Evo), Prayer (will need Mythic Spell Pen anyway)
Lvl4: Resounding Blow (Evo), Forced Repentance (Enc), Chains (Conj)
Either Evo with Favorite Meta Persistent or Azata for Favorable Magic and Enchantment boost could do ok. Whisper is AoE Sicken + +2 Sacred bonus for (Good) team, Arrow dazes Demons with decent touch damage against them. Obviously getting Archon's DC high is good. Resounding procs Stagger every hit. Alternatively Azata could use Challenge, Litany of Eloquence, and Forced Repentance with high DCs to debuff/take out key targets. Don't need many spells if you've got good ones that can land but that's an uphill battle on Pal.