Can anyone point me toward a resource which discusses whether or not moderate to moderate-serious stuttering can invalidate the essential form of a sacrament, thereby rendering it invalid?

If poor pronunciation affected validity, then Cardinal Cushing and his Boston honking/excited goose noises through every liturgy...

What about:

I baptize you in the name of the Father OOPS and of the Son...

And

I baptize you, sorry buddy, in the name of the Father...

?

Pohle-Preuss, The Sacraments: A Dogmatic Treatise (vol. 1): The Sacrements in General, Baptism, Confirmation:

Right, as long as the meaning remains clear, there's no issue with some repetition, hesitation, and the like.

In English, for instance, it might be ... "I ba-ba-ba-pat-ta-ta-tize you ..."  Meaning of this is still clear to anyone who hears it, that it's a stuttered up form of "baptize".

But wouldn't a priest, whose speech were so impaired that he could not pronounce the words of the various sacramental formulae correctly, be irregular for ordination?

The question that then begs to be asked and answered (something entirely different from "begging the question", petitio principii), is whether a priest who was ordained not having such a defect, incurred it some time after ordination.  I know that Newchurch bends over backwards to accommodate situations where the priest cannot speak, such as dreaming up ways for nonverbal autistic priests (again, this should have been caught prior to ordination, you don't "become" autistic) to offer Mass, and then there is that travesty of a Mass celebrated using American Sign Language.  Newchurch is so desperate for priests, that they will find some way to ordain anyone... unless, of course, he wishes to offer only the Traditional Latin Mass.

I, myself, would probably be irregular for ordination (even if I weren't married in the Eyes of God), as I do not have the suppleness of body to be able to kneel, genuflect fully without gripping onto something, or the stamina to be able to stay on my feet that long --- I can't even properly serve Mass anymore.  (In my dotage, I have even been reduced to having to take a stadium pad to Mass to sit on, as I cannot sit on a hard pew for an hour at a time.  It's a pretty sad state of affairs when a man's butt gives out.)

But wouldn't a priest, whose speech were so impaired that he could not pronounce the words of the various sacramental formulae correctly, be irregular for ordination?

.

Yes, probably.