For me, I think the bottom line is that regardless of what the GF should or shouldn't have done, the BF's parents were very ungracious and at all turns thought the worst of her, never giving her the benefit of the doubt in any way. It's obvious they don't like her and want to get rid of her, and they may have succeeded. I cannot imagine the relation-ship between the GF and BF's parents improving no matter what the GF does now. As I've said in a previous post - No way I would ever visit their home again, and I would not advise her to "be the bigger person" in this situation.
Frankly, I think the BF is a big part of this problem. He allowed his parents to treat her so shabbily.
Also, who asks someone to call them Mum and Dad at the first meeting? Add in the probing about her disability, and I don't blame the GF for reacting uncomfortably.
One thing that I don't think has been brought up yet is the expectation that older, more experienced people (i.e., parents) should be a little more forgiving when younger (i.e., people in their late teens, early twenties) muff things up a bit. Many of us were etiquettely challenged at that age, and also intimidated by the parents of our BFs/GFs. It can be hard to advocate for yourself in that situation, which is why I'm coming down on the side of the BF being a spineless wimp.
To answer the bolded: People who believe that their son is bringing home the girl he's going to marry to meet the family. And I think this is a big part of why things just aren't working out right now. If parents are thinking that this is their future DIL, then they're likely to want to try to establish that familial bonding right away. So, imagine meeting this girl and she immediately shows discomfort in your friendliness. Spends most of time in her BF's room (even before she was in pain). What would you think? Probably that this is a woman that wants to marry your son but has little to no interest in the rest of the family. Not a good first impression.
Now, obviously that wasn't the reason that GF was there...but it just seems very very likely that this is what the parents thought. If no one explained to them that this wasn't the case and she was really just visiting BF and taking her social cues from him, then how are they to know? I think also asking questions about a disability is something that a future MIL might do. Afterall, this is someone that she believes she is going to be spending quite a bit of time with. Wondering about how permanent the injury is, what this means for her son as a possible future caretaker, how it might affect children they'd have, etc...these are legitimate concerns. I do think that the BF should have cleared up the misunderstanding about where they stand between the first and the second visit making many of these questions moot, but I also think that the GF owed the mom an explanation of how her disability might effect her stay and the hospitality offered her...because it ended up effecting it.
And I absolutely agree with your point that older people should show some forgiveness to younger ones when they don't act perfectly. I do not, however, think that excuses the behavior. The GF didn't really do anything wrong, but I do think she can improve for the future. So, while (as I suppose an older person) can definitely cut her some slack, I'm not going to just sit back and say "It's ok, you're young. Mom is horrible." There's nothing wrong with offering advice and perspective for next time. It's how we grow.
I also don't see in either the OP or the follow-up that GF retreated to BF's room without a word, without explaining that she was in pain, etc. For all we know, she could have explained very thoroughly, and the parents simply didn't want to hear it, or didn't believe her. The fact that the BF was surprised doesn't mean much to me, as others have pointed out, the experience of pain is personal and hard to communicate, particularly to someone young and healthy, who has to basis for understanding. GF could have been saying, "Look I'm in SERIOUS pain, and need to lie still," till she was blue in the face, it still might not have sunk in to BF until he saw her try to move.
However, according to the OP, the BF was the one that asked if she could stay and he only asked once. I'm still not sure how you expected the parents to understand her pain if the only person that was asking if she could stay didn't get it. I'm healthy...but I'm not in my twenties and I don't think I'd automatically understand that someone that has a spinal injury would need that much bed rest after a day of sitting around and talking. Sure, if the GF said
something to them, they'd know, but that would have been a pretty big important peice to leave out of the story, especially when people were asking her before the follow up. My guess is that she while she spent some time during her previous visit answering questions about her injury, she didn't really explain that she was in great pain after socializing and couldn't get out of bed (I'm also assuming that there was little opportunity for her to explain this. It makes sense that she went to bed that night and woke up in agony and couldn't really get up. I doubt mom came in asking about her, so all communication was happening through BF. And let's face it. He hasn't proven to be the best communicator so far in this story).
And I'm thinking this through how I followed the story. In the OP, we were told that sitting around for too long caused great pain. I believed this to be true. So, when she went on to say that GF was in great pain after sitting and socializing, I naturally assumed she sat for too long and didn't get up and walk around when she needed to. I'm assuming that the GF probably also told mom that if she sits for too long it causes her great pain. Now, the OP came back and said that she actually did get up and walk around frequently...which surprised me, because I thought sitting around for too long was the problem. But she didn't sit for too long (at least it doesn't appear that way). So, I seriously doubt that the mom was thinking "Oh, she is sat for too long" if she saw that she got up several times and walked around. Yes, the sitting and socializing still caused the pain, but I can totally see how someone woudn't connect it right away...because I didn't.
So, an important thing to say is "I'm in pain, despite the fact that I walked around and tried not to sit still for too long. I need to go bed. I'm sorry but this is the nature of my injury and I'm not purposely being anti-social." I would love to know if the GF said this and I would also like to know that if she said this, why this critical peice was left out of the story.