1) Stop looking for progress from her. You are too HER focused. You need to focus on you. If the car is in both of your names you can take it anytime you want. Period, end of story.
2) Stop engaging other people in R discussions. If you shouldn't be engaging in R talks with your wife then you certainly shouldn't be engaging in one with her mom! No good can come of it. You can't control what she does, but if she tries to put you on the phone and/or put it on speaker, just say "I refuse to discuss matters that should be private between my wife and I with others." And walk away. Your MiL might get ticked, but she will respect you.
3) Continue to be vague what where you go and what you do. She is showing interest because her curiosity is piqued. That is a good thing if your goal is R. However, don't read too much into it and just keep it up.
4) Very good response with "She asked if I was ready to sign papers and I think it surprised her when I said my position has not changed." If you are opposed to the D then do nothing to help it along save anything that the court orders or that you are legally obligated for. Make her do all the work.
5) Her calling her parents to tell on you is not a good thing. Not necessarily a bad thing either. Obviously they will always be on her side, she is their daughter. So don't read too much into anything she or they say or do.
6) In the future just tell her you are taking the car. Loving detachment, and not wanting to be passive-aggressive demands it.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018