1) Frankly, it’s not your business to go around deciding whose gerus is valid or not.
2) The Shulchan Aruch does not actually require a ger to become completely observant before converting. The practice of requiring that is very recent. It makes sense, given current circumstances. But we should keep the basic halacha in mind.
3) It’s not the halacha that if a ger ever commits an aveira the gerus was never valid. What matters is whether he/she accepted the mitzvos at the time of conversion. Eating a cheeseburger the day after the conversion, for example, is a pretty good indication that the ger didn’t accept the mitzvos. Regarding headcovering, what if she wore a scarf or something for the first few months after converting but then the yetzer hara convinced her to stop doing it, since some of her MO friends don’t cover their hair anyway? If one is observant after conversion but then becomes less observant later, that doesn’t indicate that the convert didn’t accept the mitzvos.
4) The idea that conversions can be declared invalid afterward after a convert is seen transgressing is extremely recent and extremely controversial. There is also a lot of misinformation about it. There’s a story of a rabbi who “annulled” a conversion of a giyores after seeing her wearing pants. It turns out that was a rumor and it never happened.
5) Although the requirement to cover hair after marriage is widely accepted by rabbinical authorities, its practice by frum women has varied widely through space and time. One to two hundred years ago in Eastern Europe, my understanding is that no women, even rebbetzins of famous rabbis, covered their hair. So could there have been no valid gerim during that time. because no women would cover their hair afterward? That wouldn’t make any sense. In some MO communities today, many women don’t cover their hair outside of shul. If one is just as observant as a normal orthodox Jew, but not perfectly observant on a particular mitzvah only out of ignorance (because it is neglected that community), I don’t see how that would invalidate a gerus, because it doesn’t indicate that the person failed to accept the mitzvos. Gerim aren’t required to memorize the Shulchan Aruch, its commentaries and the teshuvos of all major poskim before converting.
6) How do you know it wasn’t a sheitel? I know an Asian woman (not a ger) who wears a black wig at work (to hide her unconventional hair style), and no one suspects it’s not her natural hair.