I'm happy to SPECULATE as to what might happen here, but please keep in mind, that's exactly what it is, speculation. While I can appreciate your concerns about your attorney, he is clearly in a better position than me to address this concern.
As to that, you might want to consider changing attorneys. It sounds like you're very dissatisfied with him, YOU did the research, not him, he has done little for you as to a plea offer and hasn't followed up on the search warrant issue.
Anyway, enough preaching....
To demonstrate inevitable discovery, the prosecutor would call the lead investigator. The investigator would discuss all avenues he had pursued, the evidence he had obtained, etc. Ultimately, the prosecutor would want the investigator to testify that, through his other leads, he would have ultimately located the car and conducted a lawful search of the car.
As to still needing a search warrant, it depends. For instance, they wouldn't need a warrant to see the outside of the car and/or anything in plain sight within the vehicle. Additionally, their observations of these things (the outside of the car and anything in plain sight), might provide an additional basis for a warrant.
What will happen here? Tough to say. I would suggest setting up an office conference with your attorney. Discuss all your issues/concerns with him. After the meeting, make a decision as to whether you stick with him or hire another attorney. Assuming the offer is still the maximum, you've got nothing to lose in pursuing the motion.