I think that it's pretty clear now that your mother isn't an accurate source of information about any of this. I get that what she says has a lot of weight with you -- of course it does, she's your mother -- but anything she says about any of this continues to come from a place of complete ignorance since, to my knowledge, she still hasn't made any effort to learn.
I think that trying to think of what might happen, or what you might want years down the road isn't very useful.
It's not petty for you to want your gender affirmed. It's not selfish for you to have your own gender identity and for that to be different than the one your mother wants you to have. Would it be so for your mother or grandmother to want the same for themselves? Just because they're cisgender and you're trans doesn't make any of this more valid for them than you.
I've been a little out of the loop with you lately: were you ever able to get connected with a more local resource to help you out? If not, can we give that another go? It's clear you remain feeling very isolated at home, and it really may not need to be that way, at least not to this degree.