So a bit of a devil's advocate - when my dad passed away, my mum kept using his car but because it was registered in his name, and because the registration document was in his name, we ended up in an idiotic and bureaucratic situation where the car was due another annual inspection, but the registration document ran out of spaces for stamps the year before. Normally, this isn't an issue, you just go to the government office and they give you another blank registration document with more room for stamps(the stamps prove you had the car inspected basically). However, because the owner of the car(my dad) was deceased, there was no one who could do this, and the government official refused to issue the document to my mum as it wasn't her name on the document, not without the court documents proving that the inheritence proceedings have been settled(which took another 6 months). Yes, she could produce the death certificate, but that wasn't enough. So she couldn't get the car inspected, and therefore wasn't legally allowed to drive it.

I'm almost certain a car dealership would accept a death certificate if she needed another key made, but my point is - yes, even with an official document stating who is the owner, you can still end up in some weird edge case scenario.