Regarding an estate: Can a companion to the deceased take possession of his car (on payments in his name)

A couple things you may not know that can be useful: your uncle's power of attorney is not operative after his death, property he owned jointly with his companion goes to the companion automatically upon his death regardless of what his Will states, and any assets owned by a trust are managed and distributed in accordance with the trust not his Will.  An executor of your uncle's estate would have the legal authority to control and distribute any assets owned soley by your uncle in accordance with his Will.  The probate court appoints the executor, so the next step would be to file with the probate court.  Spouses, parents and adult children have priority to be appointed, but your filing as a niece or nephew would at least get the process started.  Notice would go to his companion and closest relations, and they would have the ability to seek appointment and file his Will, or be required to file the Will with the court if they do not comply.  If the companion seeks appointment, you can oppose it and present evidence to the court of her or his taking the car, etc.