By 12th level, Mithral Twilight armor is affordable, and a 3rd level spell slot for Greater Mage Armor is pretty trvivial, too (more painful is wasting the spell known if a sorc, actually). Don't see why "casting arcane spells in armor" is so important in figuring this out anyway. If it is, though, I guess do Warmage//Cleric or Warmage//Favored Soul. In either case, max charisma, only fll up wis enough ot cast spells. If a cleric, use the divine spells for buffs / spells w/o saving throws so your low DCs don't matter.
Another option for one side is Warlock. Can use invocations in light armor and many of them are passive buffs, so it'd work decent with a more active casting class on the other side.
If doing a lot of spont. casting and metamagic, definitely get the Rapid Metamagic feat from C.Mage so the casting time is not extended.