Have to admire the way you're handling all this. I mean this is what life is all about. This is religion in its reality. From a certain perspective you cannot fault God and divine wisdom for giving such people of faith offspring who just happens to be trans.
It never ceases to amaze me the amount of people who quote from the Bible and miss out on the quote of quotes, the mother of all quotes, the one which captures the very essence of Christianity and what it's really all about.
I'm referring of course to John 14.
This is the stuff Jesus said about 'I am the way, the path and the truth..' Here he was being very clear in that being a Christian isn't just about being able to talk the talk, you also have to be able to walk the walk and do all the stuff that Jesus did in his life such as standing up for the prostitutes, supporting the homosexuals and helping the poor, the sick and the needy.
The prophet Mohammed was a bit more blunt on this subject.. 'Before you learn to love God you must first learn to love your fellow man.'
I can't say I've read all the Bible or even that much of the Koran, but I can't really recall reading anything in any of these scriptures about Jesus and his disciples persecuting gays or or harrassing trans folk.
Therefore I can't really understand why your mother has decided to go all Charles Manson on you just because you appear to have left the brotherhood of their cult of the self.
But on the more positive note your parents are supporting you in a way which is going to help you in your transition and that is by inspiring in you great fortitude and courage. This is all you can really do in such situations, and that is when such people aren't as forthcoming in their support or acceptance you have to exploit the situation and relationship and come up with whatever positives you can find yourself.
I know that it must be draining for you, upsetting at times, but you're doing all the right things. Besides there's none so blind as those who will not see.