Are you aware that the Embassy staff and the Immigration staff are totally different and each has their own rules and advice. No surprize that Immigration act different to the advice you got fro, the embassy.
When I purchase a vehicle, I tell the previous owner that they need to accompay me to the motor registry office and that I will pay once the vehicle is in my name. Never had any objection and always worked well.
Your first step is to break the link with your original O-A visa. Let your current visa expire. Leave Thailand and return on a tourist visa from a Thai consulate in either your country of passport or a near Thailand country. Once you return to Thailand start the process to apply for a retirement visa from you local Immigration Office. No medical insurance required but you will need to season 800,000 Thai baht..