What a joke. You said it yourself that you didn't even use any attack magic. Seifer CAN'T win even IF you had used it. Since you gave an expalanation of Seifer's abillities, I'll do the same.
Seymour-Fire, Fira, Firega, Thunder, Thundera, Thunderga, Bilizzard, Blizzara, Blizzarga, Water, Watera, Waterga, Break, Protect, Shell, Reflect, Dispel(ALL), Multi-Magic(Fira, Thundera, Watera, Blizzara), Lance of Atrophy, Flare, Cura, Ultima
Mortibody-Desperado, Cura, Shattering Claw
Mortiorchis-Cross Cleave, Full-Life
Seymour+Mortiorchis-Total Anihillation
Each of the attacks listed above are extremely deadly. Seymour and Mortibody's Cura could easilly cure Seymour's HP up by 3,000 combined. That may not seem like much, but Seifer can't hardly hurt him anyway. Don't get me wrong, Seifer is strong, he's just outclassed in this instance.
The Lance of Atrophy causes Zombie, but it can still deal some good, physical damage.
His most powerful attacks, Flare, Total Anihillation and Ultima, could probably combined take about 1/3 of Seifer's final HP. Total Anihhilation is an attack that causes multiple damage. At first he would probably cause 2,000 damage, but when he launches himself into the air he would probably cost 3500 damage. Ultima would cost Seifer well over 7,000 even though his Magic Dfense stat is somewhat high, it isn't high enough.
Cross Cleave would hurt him about half of what Total Anihillation could do.
Now onto Defense. Seifer with Gunblade Slash wouldn't even reach the 900 mark. His other powerfull Limit Breaks could cost about...2000. As I said above, Seymour's physical defense has pretty much always been high. Seifer's magics are just there to keep him interesting. They're really crappy and pathetically weak.
As for Anima, his HP is rather high from what I remember. Seifer would get somewhat hurt, but he probably could kill Anima.
Other Issues: Seymour's love was even more than obsession. After death, his plans changed and so did his personallity, power-hungry to destructive. Though his love for Yuna never lessened. It was a mortal desire that could not be stamped out by the cold hands of death.
Battle:
Seymour: Casts Protect
Seifer: Dispel
Seymour: Thunderga
Seifer: Gunblade slash
Seifer: Slash
Mortibody: Desperado
Seymour: Blizzarga
Seifer: Firega
Seifer: Slash
Seymour: Break(Seifer can't be Petrified)
Seifer: Tornado Slash
Seymour: Cura
Mortibody: Cura
Seifer: Cura
Mortiorchis: Auto-Mode
Seifer: Thundera
Mortiorchis: Ready to Anihilate
Seifer: slash
Seymour: Reflect
Seymour+Mortiorchis: Total Anihillation
Seifer: Bloodfest
Seymour: Flare(on himself but he cast Reflect on himself)
Seymour: Lance of Atrophy
Mortibody: Shattering Claw
Seifer: Cura
Seymour: Ultima
Seymour: Firega
It would go until Seifer was dead. Seymour can cure himself and his attacks are three times as powerful as Seifer's attacks.