Okay, so now we have Rav Yaakov Kaminetzky paskening like the Mishna Brurua.
As far as b’ula being required to cover her hair but a rov wouldn’t be so “heartless” to enforce following halacha, that doesn’t cut it. We don’t throw out halachas because one feels the halacha in “heartless”. Note, I am not saying there isn’t legitimate heteirim, but that can’t be it. In fact another poster above said she was actually *married* before she became a BT and Rav Shlomo Zalman said she could still leave her hair uncovered, even though not only was she a b’ula but she was actually previously married — albeit without kiddushin. So perhaps there are shittos that don’t hold of any halacha that purportedly requires a b’ula to cover her hair?
And does Rav Moshe address the issue of almanas and g’rusha’s covering their hair in the Igros Moshe? Or where does he, or other poskim, address it.