I havnt read everything i mainly skimed the post.
Well here a few thing i wanted to say.
Summon do take lot of concetration and some time to cast.
For spells i beleive it should be based on how skilled the caster is and the level of the spell. Here is an example of what im saying.

In Chrono Trigger
Crono and Magus both have Lightning 2.
Magus being more skilled in Magic would cast faster and deal more damage then Crono.
Also it should take less time for Magus to cast Lightning 2 then Dark Matter.

There level of the Spell should also been taken into account.
Let s take Vivi from FF9. True he is a wondeful and extremly skilled Sepllcaster ! But in a Zelaron Melee i doubt it should be instant for him to cast Doomgaze ! While Meteor is also a strong Spell Vivi got with his skills it wouldnt be instant but yet wouldnt be long to cast

Well Raziel let me know if you disagree with any of this.

So Seymour being mostly a above average Spellcaster and a ok melee(i think) would be good at casting his spell but nothing exeptional.
From what i seen Seymour isnt strong on the magical resist side but more on the physical def. and that would give the advantage to Seymour over sigma. Anima look more as a pokemon then anything else yet it still could do damage(minimal maybe but still dmg) and would give time to Seymour to understand more Sigma attack pattern.