THIS IS THE KEY. If only this were preached from the pulpits from every Trad priest. This is the true answer, based on canon law. It's not an opinion; it's not a guess; it's canon law.
Right, and I believe it's based on some very authoritative papal teaching.
As I said, the only practical difference between certain invalidity and positive doubt is in danger of death situations, where in the latter you can attempt to receive Sacraments from a doubtful priest if that's your only option, whereas with certain invalidity you can't. If I were dying and there was no other chance to have a priest do anything, I'd go ahead and avail myself of an NO priest ... just in case. Who knows, even?, maybe they were transfers from an Eastern Rite, or something. I pray it never comes to that, but God's will be done. If God wanted me to receive Last Sacraments as I lay dying, he wouldn't send me an invalid priest. If he didn't want me to receive them, then He would. It's entirely His will. I hope that it's the former due to First Friday promises.
But it's the same for me about Baptism. You could always wonder if the priest who baptized you may have botched the form. Well, if that happened, it was God's will. God could have willed for me to be born among animists and never be baptized at all. Salvation is a free gift of God that I don't deserve. We can pray that if that happened to us, it would be discovered somehow and fixed or that God might send an angel to baptize us, or as in the case of St. Peter Claver, have a saint raise us back to life.