The writers told us she's an expert. Beyond her initial attempt to coax Khan into talking about the 20th century, I wasn't convinced she was anything but a semi-talented painter who was far too emotionally invested to even begin to be objective (real historians do need to be objective and follow the facts, not their personal biases).
Exactly, and when the writers tell us
something like that, its usually true unless by the end of the episode it's proven wrong. Not by the writers having the character do stupid things, but by the the characters discovering the person is a fraud. The writers were working from a 60s view of women that includes "female urges" and a desire to find a man. So any short comings Marla has are laid at the feet of the writers, not the character. We can't rationalize that away.
It doesn't have to be 100% loss; just enough information to give future historians an incomplete or incorrect conclusion would be sufficient for Marla to make a mistake.
Yes, but that loss is a pretty significant one. One that includes all descriptions and images of Sikhs as well as texts describing them and their cultural practices.