Good advice here. Listen to it carefully and go with your instincts. One word of caution: do not send the elders, or anyone, a letter expressing your deepest feelings. You will not sway them. It will be used against you. Sounds like you wrote a letter to yourself and decided not to send it. That's fine, but no make that decision irrevocable - tell yourself there are no circumstances under which you will ever show that letter to anyone connected with the organization.
I like the advice you got along these lines: You owe them nothing, don't give them any power over you, be polite and cordial and just say you are working these issues out and you will call on them for guidance if and when you feel the need to do so (but not before). As you get stronger, believe me you will never feel the need to call on them.
Your conduct in this trial, and it is a trial for you, could someday impact your mother in a good way; you never know. Be strong.
Finally, let me pass onto you what I learned in my planned exit from the JWs after nearly 30 years (more than 20 as an elder):
Once you learn that the JW "Truth" is not the truth, you have a crisis of conscience. You either have to act on it or stifle it and go on living a lie. If you follow your conscience and walk away, you forever hold the moral high ground. That means you are morally superior to those who know what you know but do not follow their own consciences and leave. And this especially includes the elders, whose job it is to enforce your compliance with dub-think. They will never have your best interests at heart because they are trained to ignore and denigrate your ever increasing pangs of conscience.
When they talk down to you it's because you hold the moral high ground. As the morally superior person in the discussion, you are able to see that and simply show your inner strength and be done with them. Do not let them set your agenda for you.