You misunderstand Diego. He isn't keen on zombies, he is just trying to utilize the covenant's assets as best as he can. Portinor's character is into necromancy, and undead troops don't have to be fed or paid 
Undead troops also can't man the walls unless you want to cause disturbance, nor can they accompany a mage into a town or adventure without causing a major stir. All the arguments you had against our current turb members apply many times more to undead. At least our current turb is loyal and can improvise in case of trouble.
I have a plan for the current turb . Will send those notes to the group later.
But anyway, undead troops
A) are a last resort
B) don't have to be mottled corpses; they can be fresh and clean and dressed normally, covered up no one will know the difference.
C) don't have to be dead people, as you can use magic to create a fresh corpse from scratch, without mark or blemish.
Yeah, a Creo Corpus group to summon 10 zombies means you have armed forces on call. As a player I love what a good necromancer can do. Just need to make sure the neighbors don't know.
Mostly waiting for a really great scourge of plague then gather as many as I can in my little hiddy hole.
Though I still like the idea of going around to those who have leprosy and offering to kill them then bind their souls back into their corpse. In return they gotta work for the magus for ten years.
Necromancy is a way of life that is simply concerned with economy of resources. L The ultimate in practicality.
As Kallista said "Reduce, Reuse, Recycle." Necromancer Credo
Sums it up perfectly.
How the heck do you sell that? "If you want me to insure your Damnation, you'll owe me ten years of service. You could just kill yourself and be Damned, but then I'll have no way to torment the people you loved. I'm sure you can see the valuable service I'm providing!".
"You are damned and cast out. The disease of leprosy that afflicts you is the sign of your damnation. Agree to serve me and I will make your bodies whole and insure you do not know hunger, cold or pain again. You will not see yourself degenerate. You will die and for ten years you will serve me in your perserved and healed corpses and then you will be set free. You will forever be free of the fires of hell by my magics."
My goodness, your group is begging for Divine and Infernal problems, aren't they. And possibly a Crusade and Wizard marched to boot. I mean, tempting someone to commit a Mortal Sin, and claiming that you can overrule God and redirect Souls......
Hey, you asked how it could be argued. I am saying how it could be done. My character prefers people to be alive. She is bonisagus mentem researcher and corpses have no minds for her to manipulate. Just because I can come up with the warped and evil tricks (that I know will be used in the game to the horror of my character) doesn't mean I support it.
Still who notices what happens to lepers and if they wander off to suicide and be turned into reveenants, there will be no witnesses. So the necromancer and the practical flambeau might get away with it until the others in the covenant find out and are horrified. Even then, as long as there are no witnesses and such, there are limits of what they can do without weakening their Sodalis's power.
YEah, This is a covenant wide open for diabolic temptation or holy man with visions thanks to those two but that that is part of the game. The fact is that suicide leading to a ghost and then binding that ghost does mean we are doing what was promosed. It is off the topic which is how to get the corpses, how to animate them in en mass and the most efficient means to do so.
I don't think you have done "what you promised", unless your game has allowed or researched a way to 1. Make Ghosts (something that in canon must be researched as a mystery, and can only be done to oneself) 2. Effect the Soul, as opposed to the Spirit (which would seem to break the Limit of the Divine and Essential Nature) 3. Destroy the Damned Soul after ten years. This being the foulest thing imaginable in medieval Europe, there ought to be a level nine or ten Infernal aura real soon. The only real question is whether it would happen the first time, or would you get a couple of Damnation's before destroying your Magic aura.
I just fell on the floor convulsing with laughter... my my... I can only imagine the Divine and Infernal showing up at the same time looking at each like "You got the memo about these jerks as well?" It would be the first time since Creation that they ever agreed on anything.
"Ars Magica. We bring the Divine and Infernal back into accordance."
OF course if they get the souls bound, they would would not release them after 10 years. More like never so until the container is shattered, the ghost would be safe. Now admittedly they might not be happy about it but well once in the amulet, it is too late to have regrets.
YEah, I can see an infernal aura starting to grow on the side of the large hill hill that is the covenant's magical aura. What makes it worse is that at the last covenant council meeting, they voted the mentem maga out of being regent to keep eye on things in favor of the auram mage.
Probably cause a civil war in house Criamon as well. A destroyed Soul has, after all, left the Circle. Acting in Harmony or not? There would be riots in the streets, as it were. Not a dull game to be sure. Destroyed your home Aura, welcomed nowhere, may have destroyed the Order, but never a dull moment!
Real Evil uses children as human shields. Zombies are child's play, no pun intended.
Steal children and mind control them to protect you. When they die, send the spirits screaming they were innocent souls, cripple your enemy with guilt. As you animate the children corpses... Makes it easier when you have your enemies kill themselves.
You are not helping. My character is the covenant mentem expert who has her own warped nature. She is a firm believer in a sun duration WEight of 1000 hells (flexible formulaic magic is fun) and has spell to control a target's mind to make them her puppet.
I've thought about that spell... It's a nasty one. I've been thinking the opposite of that spell may be of really good use too. That is give the gift of Heaven to a person, then rip them out. After a while they'll beg for more. Either way they just aren't the same afterward.
Since I play generalists a lot and love illusions i wouldn't have that spell... but if i were in the campaign I would plan on making the enemy look like zombies too to confuse them.
Back on topic... Zombies 
It may be a good idea to have the corpses explode when injured too badly, a short duration spell store that is not a ritual would do the trick. Shock and Awe brought to a whole new level.
Still not indication that they've figured out a way to use Magic to affect souls. If you are going to allow something like this, you should have them research a mystery to do so. The easiest thing to do would be steal the Mystery of Awakened Devices, with the limitation that the skills and stats are opened at the level of what "the body" had, as a fixed enchantment, maybe level 25.
Funny that you mention this. When we found Portrainator's character he was making money selling abortion potions...
I think Belanor (P's character) should have his own necromantic Kuato (see Total Recall).