I wouldn't use game stats to say no stormcloak/imperial wizards or all altmer know magic.
Learning magic means learning the mindset of each. For some learning magic is even impossible. Some people are better suited to restoration, others illusion, and so on and so forth. Hence why you only tend to get trainers of one skill.(Unless we're talking morrowind where they'll teach you up to their 3 best skills) Some people are suited to one school more than others but are still mediocre in ability. The battle of red ring took out a lot of the destruction suited ones.(Possibly restoration types too) Even then, with what we've known from each game, it takes MANY years for them to reach a respectable level of competance. This is of course leaving out the PC who is always wildly adept at magic.
You do realize how large the imperial city is right? They had to form a ring around that. That's a dangerous manuever that spreads your forces thin. So long as there aren't enough outside forces or the enemy doesn't make a sudden surge at a weak spot, you should be fine but if they do manage to break your line, you run into some serious issues.
What you're suggesting is that the thalmor had a larger force waiting in reserve that hasn't been utterly exhausted by battle like the empire, and decided not to crush the empire and take the IC with it.
The Dominion had:
1. A home guard.
2. A navy (which was never defeated)
3. Forces in Hammerfell. A substantial minority of their invasion forces, which they were allowed to withdraw after the second treaty of Stros M'kai
4. Forces in Cyrodiil, occupying the at least three other cities (Leyawiin, Bravil, and Skingraad were certainly occupied, but there's no definitive list) that were certainly hostile to them, and required a significant military presence, which they were allowed to withdraw after the White-Gold Concordat was signed.
5. Forces occupying the IC at the time when it was suddenly attacked and taken in less than five days.
Only #5 was destroyed. #3 was heavily harassed and forced to retreat on occasion, #4 faced opposition but always overcame it, and #2 & #1 were always safe (#2 saw battle, but never lost).
The sum total of the troops in #4 and #5 likely exceeds the number of troops in #3, but the number of troops in #5 alone is almost certainly less than the number of troops in #3. That the Altmer weren't able to gather an army large enough to swing the battle in their favor doesn't mean they didn't have those forces, it's merely reflective of how amazingly fast the Battle of the Red Ring was. It lasted less than a week. That's just not enough time to gather those forces.
You know the heart of mankind.
lthe empire(Who dropped down to less than half its forces with a smaller army than this reserve that's waiting around)
1. A reserve, which was larger than:
2. The legions, of which less than half remain.
I don't see any reason to believe this, and, furthermor, it makes no sense. Why wouldn't the empire call upon these reserves during the months when their homeland was taken from them? Where would these reserves be located? It can't be Cyrodiil (It was otherwise occupied), it can't be Skyrim (the civil war would make no sense that way) and it can't be southern Hammerfell (it was otherwise occupied). Does the Illiac bay contain some huge troop reserve for some strange reason?
*Seriously, at this point I have no idea what to make of the IC, especially since both of the times it's traded hands in The Great War make no sense if the game is to be taken as an at all serious model of what it looks like. Instead of surrounding it on all sides, why didn't they just occupy the one big bridge to the mainland. How the hell did either the Dominion or the Empire storm the incredible island fortress we see in game? How the hell was either conquest of the IC a battle, and not a protracted seige?