Tough situation. You have my sympathies.
Its as hard to give sensible advice as it is to take it. Well not quite so hard maybe, but hard enough to see things through your eyes.
One thing to seriously consider is what your mom will do when she discovers you intend to officially quit (I don't use either D word). The dubs seem to be going through another of their 'paroxysm of paranoia' spells and there is pretty strong pressure now for good dubs to stay away from the fallen - even family members. How would your mom respond to pressure to not let you live with her? What would you do if she said you can't stay if you've quit?
As to arguments with mom, family and other dubs, well there's a saying 'argue with a crazy person and you lose...even if you win, you lose'. You can substitute JW for crazy in that saying and miss nothing. Generally, all a 'discussion' will do is make you upset and probably confused at which point your mom may sense she is making some headway and really go for the jugular. (The reasons for the confusion have all to do with dub rhetorical practice and little to do with your own sense of logic and reason - a whole other topic.)
I'm with rebel8 on this one. If you decide you need to officially quit for your own peace of mind (which is what I did), then simple is probably best. "To whom it may concern, I am no longer one of Jehovah's witnesses. Signature". What the dubs do with that is their business...but maybe your mom's as well. If you don't need to formally quit right now, then it may be best to slide along (which is what my son did) so you can keep body and soul together till you can move out. They finally kicked my son out, but by that time it didn't matter so much. Surely, you have developed some techniques for appeasing your mom without giving in totally. If not, best plan on making the home stay very, very brief.
Abraham Lincoln coined a wonderful phrase when he didn't want to argue and wanted to appear to agree with some ludicrous statement. He'd say "I shouldn't wonder" in a voice full of emotion and nod or shake his head. The information content of that phrase, however, is zero.
Good luck with it all. In one fashion or another, most of us on this site have faced a similar situation, so you have our understanding, although trust us, the hard part - quitting in your own mind - is past you.
Damocles