If there for instance is some überpowerful being that the characters though they could never beat, this would be a viable option to dispose of that character.
Um... okay. "Hey, that evil warlord demigod is too strong for us. Let's go adventuring for a few years, so Presto the Magician can gain a few dozen levels and learn 15 new epic feats. Then he'll have enough power to instantly kill the bad guy."

I don't think so.
On free actions: Although it's not supported by the rules, if you have free actions, even if you lose initiative, they should come before actions, as actions take time to initiate.
You are of course entitled to your opinion on what the rules should be. Unfortunately, this not how they actually work. If you lose initiative, the opponent gets to act before you have figured out what to do.
Wanna make things work some other way in your game? Fine. The House Rules forum is
this way.
You may not think it is an issue now, and maybe it isn't for the first 100 levels, but after that, then...wizards with these combos are superior to those without them.
So, by 100th level you're casting
Finger of Death a bazillion times in a round. Whoop-de-doo. By that time, I can have developed an epic spell that makes me immune to all nonepic death effects. Cast all you want, I'll still be standing when you finish.
Not a death effect? Maybe you were casting a ton of
Fireballs, or doing some other kind of energy damage? I can be wearing my Ring of Universal Elemental Immunity. You won't even singe my cloak.
Attacks do become powerful at high levels, but so do defenses. No matter what kind of spell you decide to memorize 58 times, it's possible that an epic-level target is entirely immune to it.
[Edited to add: Even before epic levels, a simple flesh golem would stymie you. Fire and cold only
slow it; electricity heals it; it's immune to all other magic. You can't kill a flesh golem with only spells, no matter how many you cast at it.]