But no one would know it. So, it seems this whole exercise is moot.
Au contraire: All theologians today hold that the performance off the prescribed rite render the
presumption of validity morally certain.
Moral certitude is all you have ever had in the sacraments.
For example, you have always known that it was possible the priest at Mass feigned the sacrament (e.g., by deliberately omitting the essential form of the rite), but this has not disturbed you. Nobody would take such a fear seriously, even though, technically, it remains possible.
Same thing with intention.
See final sentence of this chapter by Msge. Pohle on this subject (p.187): https://archive.org/details/sacraments01pohluoft/page/n195/mode/2up
See also the quotes of l' Abbe Hunter at the beginning of this thread.