Hi Kim!
Just been reading all this through and i have no experience what so ever with kids or step parents etc but I think you are the best mum that this girl has and you are doing a great job. Grant really should try and support you more.....united together and you could get through the barricade and get somewhere together.
I would not give her driving lessons....big mistake.....never teach a loved one how to drive...learned that the hard way...that is why i still have to get my car licence! Dad (god rest his soul) tried to teach me to drive....gave up after i tried to kill us both...got a wee bit confused with brake and go faster pedal!!!
It sounds to me as if you have tried everything to get this very ungrateful girl (and she is ungrateful) to listen, understand and co operate. So maybe it is time for tough love.....just tell her that you love her as if she was your own daughter but now she has to find her own way in the world and just leave her to her own devices. Let her make mistakes and just be there if or when the shit hits the fan and more importantly get Grant to be on the same wavelength.
Not sure how easy this is to put into practice but maybe it is time that you just kick back and just say fine you know best....go ahead but I will always be here for you and you know where i will be if you need me.
Is that any good Kim? Mmmm now i see why i don't have kids!!!!
Good luck and will be thinking of you.