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This subject has nothing to do with jurisdiction.

There is no such thing as an "invalid" exorcism.

Nor is the effect of an exorcism intrinsically tied to other things the CMRI priests are doing religiously, etc.

It does have to do with having permission, so the exorcism might be sinfully performed by a priest and thus be "illicit", though it still can have its good effect.

Epikeia is what comes into play here. It means going against the letter of the law, but virtuously performing the spirit of the law by necessity. That necessity can only exist if the legitimate law giver is inaccessible and there is an immediate need.

This subject has nothing to do with jurisdiction.
 There is no such thing as an "invalid" exorcism.

In todays world to have Valid Priests who have the ability grace and strength to take on the devil in an exorcism is a big plus for us who are trying to live the Faith.  
What does that say for the "Sede vacante" position, I think a lot.
Having lived with Bishop McKenna for many years, and being considered a friend of him , attending his Mass s , I was piviy to what was going on around his circle, of which you never got from him. but from other sources.
Bishop Mc Kenna was a very holy Man, you would never know where he was , and usually found him in the Chapel praying all night, and fasting extensively before any serious exorcism.
He would take on the hopeless cases, of which no one else had any success with.  He wasn t 100 percent in all his endevors, but usually because the party wern t Catholic , or didn t want to live a Catholic life etc.

In this photo , this is a man who agreeded to be tied up, and go thru the exorcism, with a goup who brought him there.  He was put into a straight jacket, and tied to big eyelets in the floor beneath the pew.  When Bishop started his prayers , and routine, This nice gentleman became like a beast with all the foul mouth cussing etc. I ll leave it that.
The point , it takes a strong holy Priest to perform these, and they are very exhausting. Most go on for three or four hours.  We need to pray more for our Priests.