Ahh, that idea for Prestige classes makes sense. Quicken spell is actually a highly tempting feat... the idea of hitting some creature with multiple evil spells in one round is somehow appealing to me.
Seems it'd be best to hold off on Archmage for one more level, at least 'til we've gotten ourselves buffed up in case we run into any more underrated encounters.
You're right about not using those two Metamagic feats... when I selected them I was going for least increase in spell levels... I could use the Limited Wish thing to swap them out for item creation feats, which, given the current state of the party, seems more useful.
So.. take a level in Wizard, end up with feats:
-Quicken Spell (levelling)
-Craft Magic Items and Armor (levelling)
-Heighten Spell (limited wish, useful for getting the DC up)
-{ItemCreation or MaximizeSpell} (limited wish, not decided which yet)