I don't think the Musgroves (with the exception of Mary) treat her as harshly as you describe.

In Chapter 6, we are told that One of the least agreeable circumstances of her residence there was her being treated with too much confidence by all parties, and being too much in the secret of the complaints of each house. , which, whatever its downsides, is not ignoring her or casually using her.

In that chapter it is also mentioned that she sometimes walks with the Miss Musgroves and without Mary: And one day, when Anne was walking with only the Miss Musgroves ..., at which point one of the Miss Musgroves compliments her while complaining about Mary.

We are also told in Chapter 6 that the Musgroves only think of her performance out of civility, or to refresh the others. If it was just to refresh others, that would be using, but "out of civility" implies an attention to Anne as a person, even by people who cannot appreciate her musical skill. And further on: Anne, very much preferring the office of musician to a more active post, played country dances to them by the hour together: a kindness which always recommended her musical powers to the notice of Mr. and Mrs. Musgrove more than any thing else, and often drew this compliment -- "Well done, Miss Anne! very well done indeed! Lord bless me! how those little fingers of yours fly about!" - here it is Anne preferring to play rather than to dance.

Or in Chapter 7, when Anne stays at home with little Charles, her brother-in-law shows attentions to her:
"This is very kind of Anne," was her husband's answer, "and I should be very glad to have you go; but it seems rather hard that she should be left at home by herself, to nurse our sick child."

Anne was now at hand to take up her own cause, and the sincerity of her manner being soon sufficient to convince him, where conviction was at least very agreeable, he had no farther scruples as to her being left to dine alone, though he still wanted her to join them in the evening, when the child might be at rest for the night, and kindly urged her to let him come and fetch her, but she was quite unpersuadable;

I think the point about her never dancing has a different meaning than that she is unimportant. I understand that at the time dancing was quite related to courting, and a single woman who did not dance was indicating that she was withdrawing from the "marriage market".

As for the Miss Musgroves slighting her when they stop by to invite her and Mary for a long walk, we are told She tried to dissuade Mary from going, but in vain; and that being the case, thought it best to accept the Miss Musgroves' much more cordial invitation to herself to go likewise, - in other words they wanted her much more than they wanted Mary - the opposite to the interpretation you put on it.

And small two year old boys are happy to crawl over relatives regardless of how other people treat those relatives.

The Musgroves are not highly skilled at how to treat people, they are too preoccupied with their own lives, perhaps they are too fast to admit Anne as a family member (but she has been staying there before), but they think more of her than as someone to be ignored or casually used.