I won't touch #'s 1 or 2, but as for #3, it depends entirely on the priest. If he has Strong Will, or some other Virtue (like Driven?) or Flaw (like Proud?), or an appropriately strong Personality trait, he might push on with a light wound. If he has a Medium or worse, it's harder to see it happening.
Then again, it depends what kind of "wound" it is. If made by an edged weapon, it's bleeding, and no one is going to conduct mass while they're bleeding. Otoh, if he was merely pummeled, then he might "tough it out" - tho' even then, there's a limit to everyone's tolerance for such.
If he has a jr. priest that he trusts (or who is themselves Strong Willed or Proud, etc), they might take over.
I don't think there is really an 'Ars Magica' answer to these questions; it's up to the SG. That said, to my very limited understanding,
While it usually amounts to nothing, the arrested ritual means that the people there would not be blessed by it and that it will not count towards increasing the area's aura. If the interruption was horrid enough, it might even corrupt the church to some degree, but I doubt it.
The main game-effect that I can see is delaying someone from enjoying the blessing of God for some time, or preventing the Body of God of coming into being in that place for some time, or so on - doubtless that would fit well into some infernal scheme.
I believe only ordained priests can conduct mass, so that I'd say it would simply have no effect. I might let her roll some dice and, if they're high enough, have a minor miracle indicate God's pleasure, nonetheless. God is pleased with any ritual directed at his worship, as far as I can tell from ArM material.
3) If the priest only had a light wound, should he be completing mass after wound tended or just retire? What about Medium or heavy?
Totally up to him, but I'd think anything beyond a Light wound would prevent virtually anyone from completing the mass. Most will also not do it at Light, unless there is some obvious urgent need - holding a (larger) mass once he's been patched up, next week, seems a far better course. No rush.
Aren't there other ordained priests there, to complet it?
Depends on particular cause of the interruption. If I read you right someone has interrupted mass and assaulted the priest. I think appropriate results for the interrupters could range from nothing to divine disfavor, all the way up to the proverbial bolt of lightning. I do not think God would punish the mass attendees in any way, unless they practicpated in the sacrilage.
Only a priest can perform mass. Even if the holy maga is literally a saint, she could not (and IMO would not).
I'd say it depends on the wound, the priest and the mass. Generally speaking, though, I think most clergymen would retire rather than bleeding all over the altar.
Depends on the church, I think. Mass at a cathedral would probably have several priests in attendence. A rural church, OTOH, likely doesn't even have one full time priest, but rather a shares a priest with surround communities (assuming they have a priest who can perform mass at all).