Part of the reason many people just didn't believe that AA was FS was because FS was a "peasant and factory worker" and no way could such a person have acted like a noble person.
Well, first of all, FS was not a peasant. Her family was not poor and owned a fair amount of land. She didn't live out in the country, isolated from gentry, but rather had plenty of access to people of different classes. To behave in a genteel manner is not reserved for the upper classes, and I have no doubt that many "peasants" could and did come across in a cultured way. The comments by Xenia and others about her demeanor simply reveal the imperial snobbery that existed at the time.
As for being a "factory worker", she didn't work at the factory for very long. During WWI, many emigres had to resort to manual or industrial labor, and I daresay there were many people of upper classes working around her. (I wonder if anyone has investigated that aspect of her life at all?) She could have learned a lot at that time in her life about how the "other people" act.
AA/FS had plenty of opportunity to conceal any behavioral errors with her illnesses and depression. She didn't have to talk if she didn't want to (or couldn't, for whatever reasons). She didn't have to get out of bed if she was ill (or couldn't behave appropriately). She could run off and cry or have a tantrum if she sensed she was trapped or about to be caught. In other words, she didn't have to act the part consistently, 24/7.
And by being bedridden or unable to cope, she had plenty of opportunity to watch her visitors and learn about both them and their expectations of a GD.
Now, what I find very interesting is that her tubercular arm caused her to often go into a feverish spell where she would become delirious. This happened while she was staying with HR and also at the sanitarium when she was attended by doctors. Either they were very naive, or she managed, despite not being clear in her mind, to not let go of the role she was playing, not reveal her true self. Was this because she truly believed it by then, or because she was so strongly on guard that even delirium would not cause her to lose control totally?