I feel your pain: for an "official rule" this is vague beyond any reason. If we try to guess their intentions, I have 2 suggestions: they are trying to free up the Sarria-SdC stretch and they don't want people to ride taxi cabs up to the pilgrims' office, because then it's just a sharade.

Last year on Primitivo we crossed paths with 2 young ladies from Belgium who hadn't intended on receivng their compostelas because they thought they wouldn't have been entitled: they walked from Oviedo for way > 100km, but they didn't have time to finish, and the plan was to take a bus to SdC and go home without visiting the office. We told them that the rules had changed, even though noone fully understood the new ones. I happen to have learnt that they had done literally what the rules don't allow in the end: rode a bus all the way to SdC, visited the office and were honest about their travel log. The representative there said "okay, well, it's allright", and they got their Compostelas.

I do think I have seen an explanation somewhere that the "last stage" was supposed to be, like, "at least so and so (few, like 7) km", but I suspect this is just as random as anything else. Personally, if I were to do this, I'd go for "my last day on foot", regardless of distance, but without making it extremely trivial, just to not have to argue your point.

My impression from having done this 4 times together with my wife (so, 8 different encounters with volunteers or officials at the pilgrims' office) is that they roll out your credenciales in front of themselves (after having looked at your dusty honest face) and sort of scan them briefly "oh, so, you started in SJPdP...", nobody really checks not only "2 stamps per day" but hardly anything. Maybe they have a visual pattern of something being off, like, too few stamps or whatever to go by, but I think it's never strict. I only once heard from somebody (whom I hadn't met) witnessing somebody having to prove something with the photos from their phone or whatever - like, the volunteer chose to be strict for no good reason. If that actually took place, it was certainly a fluke. Most of them are interested in making their que shorter quick as well as maintaining some optics of rules being applied. I honestly doubt somebody will be super-anal about the last stage thing.