Oh where to begin..
Invisibility is FAR from pointless. As Shayuri pointed out, not EVERYTHING has see invis/true seeing, even at epic. Most PC's do, because most players are worried about covering every possible eventuality(though thats another rant)
The ability to cast lvl 10+ spells is granted by the Epic Feat "Improved Spell Capacity". It requires max lvl spellcasting and each time you take it it grants you access to spell slots of one higher level, for use with metamagic feats.
If by "the animal line" you mean Bulls strength/Cats grace/etc, then you are correct that the BASE spells are worse than items, but if you have access to Spell Compendium, you'll find that is not the case..
As for the cleric, I have to correct you there - Have you never heard of CoDzilla? Clerics are even better buffers than normal mages. Divine Power, Divine Favour, and Righteous Might(Just to name a few from CORE) are the cornerstone and by themselves put a cleric on par with a fighter. Then they get other bonuses and options... Spell immunity, healing, divinations, Greater Magic Weapon, Magic Vestement, and of course various attack and utility spells. I never felt that the fighter-mage was about casting a bunch of buffs and then just being a fighter, I always love the utility and options the spellcasting provided.
Why is knowing every possible opponent any more relavent for a fighter-mage than any other concept? I don't even think it's advisable unless your character's got the appropriate knowledge skills, b/c it leads to metagaming. (This fact and the fact that I know a lot about the monsters/powers/spells/etc at Epic is why I usually invest heavily in Knowledge skills at epic.)
Furthermore, I don't think it's physically possible to know every opponent. The majority of opponents in most Epic Games are NPC's rather than monsters, meaning they coulc be/do literally anything.
I don't want to sound harsh, but it seems like you're TRYING to come up with reasons not to play it.. If you don't feel like going through all the epic spellcasting work(It is very much like doing your taxes), then just make up something else. It's not like we're going to sneer and say "Oh that guy changed concepts b/c he couldn't do casting"..
If you're really worried about effectiveness, my character has several flaws.
Primarily, Dispels and Antimagic kick her ass. A lot of her defenses and abilities come from buffs. The weakness makes sense b/c she is a character who is becoming one with magic itself, and so if that wholeness is interupted, it leaves her weak and vulnerable.
She also has NO IMMUNITIES except for Disease/Poison and Crits, She's not sequestered, Permanently Invisible, or mind blanked.
Now, I could go back and rework her so that she's immune to everything and would never run into a situation she couldn't handle, but where's the fun in that? Epic Level to me isn't about creating a perfect character who has no weaknesses and can kill anything. Despite the fact that she has so many of what a lot of people would consider crippling flaws, I think she's going to be fantastically fun to play, and I think she's powerful enough to hold her own. That's all I believe matters.
And finally, Shayuri - See Invisibility has no range. But I think that's just as well, since without it your character would be nigh invincible at long range. Some of your spells don't even have an effect they can trace back to you, so you could just fly around bombarding and cackling in any outdoor encounter.