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The gemara says that someone was once being machmir for aveilus on yerushalayim on a level that he was presumed not holding at, and the (Jewish) government threw him in jail. When they found out he was greater than they had thought he was and he really was holding there, they let him out. There are plenty such examples in the gemara where it clearly teaches that one who is not holding somewhere shouldn’t act as if he is. One must be makir mekomo. Otherwise, one can do all the chumras and get carried away with himself, thinking he is perfect when he actually has loads to work on. See Alei Shur, the chapter on self-knowledge, I am only repeating what he said. So there is much basis for those who say it is assur to be machmir if you aren’t a ba’al nefesh. Furthermore, this is the psak I have been given by my rav, and so I have heard first-hand besheim both R’ Dovid and R’ Reuven, so don’t just assume so simply that R’ Moshe didn’t hold this way.
Your logic that one is a ba’al nefesh until he proves himself otherwise is faulty too, because 1) a person is not born a ba’al nefesh, and 2) most people are not ba’alei nefesh, and therefore there is both a rov and a chazaka to refute your claim. Anyone who’s learned some gemara and poskim knows these are perfectly valid means of determining the halacha.