Yes, you are right about the question of form -- I stand corrected and thank you for the citation.
But your point about jurisdiction still troubles me. I have heard of cases (some through hearsay, some actually reported in news accounts) of fraudulent priests hearing confessions, and I was always told that the absolutions were "validated" through the jurisdiction supplied by Canon Law. And these were definitely cases of men who had never taken Holy Orders, not simply priests who lacked jurisdiction.
Can you (or anyone else) shed some light on this point?
I'd point out that in these cases, if the penitent doesn't realize that he confessed to a false priest, then the sins he confessed invalidly will be forgiven as soon as he makes an act of perfect contrition, receives Holy Communion, or confesses his sins to another priest (even if he doesn't confess the sins he mistakenly believes have been forgiven). St. Thomas, in his treatise on the Eucharist (III q. 73 a. 3), writes:
Secondly, when received by one in mortal sin of which he is not conscious, and for which he has no attachment; since possibly he was not sufficiently contrite at first, but by approaching this sacrament [of the Eucharist] devoutly and reverently he obtains the grace of charity, which will perfect his contrition and bring forgiveness of sin.
Almost everyone receives Communion soon after they approach the tribunal of Penance, so the danger of damnation from such an invalid confession is practically nonexistent.