Not quite enough, I fear. I hope to be allowed to switch out some spells that are now unthematic (and much less effective) for my mage, too. (Not all, of course, he will stay with his charm/enchantment spells.)
River have been hit with a nerf Mace of the Titan since his creation... but most of the errata was needed and made sense... just hard to give up a candy once you have tasted it.
re: Flaming weapon being changed due to Pyromancer, I don't know if I believe that. See Frost weapons triggering Lasting Frost/Wintertouched. It looks to clearly be a spellcaster's effect, a reward for a spellcaster choosing cold-based spells, but has been swiped by rangers and other TWF out there for a massive bump in damage. See Radiant Mafias out there super-abusing radiant weapons, genasi blaster-wizards using lightning weapons the same way to get around the "wrong" elements for their own Elemental Empowerment feat. It just seems odd to grab the one element being used that way.
But it should have been or at least a more forgiving feat than Arcane Admixture. In 3.5 you could change all your elemental spell with just one feat.I don't see where this was ever really a thematic issue. The ability to do fire damage on every single power is not a normally thematic ability of any class in 4E. It's an item ability.
...
I'm not convinced.
In the cases you mention, having the proper resistances stops the damage.
In this case pre-errata, the attack that did 15 fire and cold damage would do all 15 points of damage to every single NPC unless the NPC had fire immunity.
If the NPC has 20 fire resistance and 20 cold resistance, this combo used to still do 15 damage out of 15 damage.
In the cases you mention, if the NPC has 20 fire resistance and 20 cold resistance, and a cold/fire attack did 30 points of damage (instead of 15 because of the extra feats), it would only do 10 points of damage. The resistances still apply as long as the monster has every needed resistance.
There was no way to negate every single case of multiple types of resistance completely with a single class feature and a single magic item like this combo. When you think of the ability to sidestep an entire subset of the defensive rules of the game, that's pretty darn potent.
That's why it is overpowered and errata-ed. It completely wipes out every single NPC set of resistances as long as the power has more than just the fire keyword. All of them. The NPC needs Fire Immunity to overcome this combo.
He originally started out going for Wizard/Warlock Student of Caiphon, but when SoC got nerfed I suddenly had to change the character significantly (this was around level 14).

Here's a quick and hopefully easy querry:
I have two PCs. One is level 4 and the other will be level 6 when this question is relevant.
Would I be able to retire both and bring the level 4 back at a level 6? I know it's possible to retire a PC and bring them back at the same level with a massive rewrite.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.