I carefully read the update...twice. I'm not saying that GF did anything wrong,. She didn't. This isn't about who's right and who's wrong. Who's a horrible person and who isn't. This is about managing relationships. The given is that BF and GF are staying together. Neither wants to do anything to change their living arrangements. So what's left?
If I assume that the mother and father are just horrible human beings whose sole purpose in interacting with the GF is to discriminate against her due to her disability and who also just want to maintain control of BF's life...if BF and GF could do absolutely nothing to clear up any miscommunication or misunderstanding that caused the behavior, etc, then I guess they are stuck. GF is going to continue to date a guy who lives with people that hate her. The situation will never change. She has to somehow figure out how to both maintain her relationship with BF while never interacting with his family. And maybe even BF needs to cut his family off completely too. Situation remains horrible forever and no one has any power to make it even a little bit better. And if that's where everyone is at, then that's it. Nothing more to do or discuss. And I'm sorry that GF is stuck.
But, I really don't think the evidence points there. GF did some soul searching and can't figure out why mom would think that the visit was more than just a casual 'hang out in BF's room all weekend?" I can't see it. How many women has shy and quiet BF brought home to stay with him? Does mom really ask all of his friends to call her "mum and dad?" Was she overly friendly just to throw GF off? It doesn't make sense. Whether or not the culture dictates this is what she should think, I think it's pretty clear that this is what she thought. She wasn't right, but who decided to explain to her that she misunderstood the purpose? This is getting off on the wrong foot.
GF also doesn't think she did anything to show her discomfort. I believe she really believes this to be true. But, chances are it's not. Otherwise, why would mom volunteer all on her own that GF could call them by their first names? Heck, even the OP said that the mother sensed that the GF was uncomfortable. Also, if the GF did nothing to show her discomfort, then why did the mother change her behavior? why not just continue to be overly friendly? One reason could be that she is just crazy and another could be that she sensed some discomfort. I'm not sure why anyone is so easily dismissing the latter.
I truly believe that GF and BF thought that they spent an appropriate amount of time with the parents. And for the purpose of their visit, it sounds like they did. BUT...if the purpose of the visit was to meet the parents (as I think mom was thinking), then no, the amount of time wouldn't have been appropriate and this is where mom may have had the feeling that the GF was ok with coming to their home but had no interest in getting to know them. Misunderstanding.
Finally, yeah, mom does seem to be a little discriminatory about the disability. Her concern about what this means for the future of her son may be coming from a place of concern for him or just from an uneducated, horrible place. If the GF decides to listen to the posters who say that talking to mom will do no good, then she just continues to be stuck in a bad sitation and has no power to make it better.
BUT, if she does decide to try to clear the air, extend an olive branch, and attempt to get mom's perspective, then what's the worse that can happen? I'm thinking the worse is that she's stuck in the same bad place that she's in now. The only other outcome is that things could get better. Considering that this is her BF's mom...and her and BF are staying together and not changing their living arrangement...I think it's worth the gamble.
Again, it's not about who is right or wrong. It's about managing relationships.