I'd say that the use of the car crash of a metaphor is highly plausible, and would probably make the song more credible, but I'm gonna take it at face value for my interpretation.
Firstly, I think the guy (I don't think that Paul is writing as himself - hence the use of a puppet in the vid and not a trace of the 'pol anywhere) lost his partner in a fatal car crash. I think perhaps she didn't die instantly but that she ended up in hospital and didn't make it. The song is about the guys struggle to come to terms with the loss of his partner (who I think may have been dead for some time now) in the face of feeling romantically towards somebody new - Sandy.
Right, now let's back all that up:
"Rosemary, Heaven restores you in life" - Let's everyone know she's dead right at the beginning, also shows us he believes in Heaven and believes that she may still be aware of everything that's going on. This is backed up the next few lines:
"You're coming with me, through the aging, the fear and the strife" - This line could also be a reference to the wedding vows:
"I offer you my solemn vow to be your faithful partner in sickness and in health, in good times and in bad, and in joy as well as in sorrow. I promise to love you unconditionally, to support you in your goals, to honor and respect you, to laugh with you and cry with you, and to cherish you for as long as we both shall live." I only just thought about this one, but everything he says 'aging', 'fear', 'strife' it's all negative stuff, and in the wedding vows you promise to stick by each other even in times of trouble so maybe they were married and he's saying that even though she's no longer here, she's still going to be with him through all the things they promised when they got married.