I'm not an intellectual property lawyer.

I think you are fine. I even believe you are fine if a car owner had that specific tag.
Since the original tag has been modified to a fictional order of letters and a number, I think the tag owner would have to prove you actually took a picture of their tag.

Since you can prove that you didn't take that image and by keeping the Photoshop file showing all layers of editing including the fact that these letters are your fiance's initials I'd say it's a far stretch to get sued over this.

You aren't spelling out a name, brand or other copyrighted words and I also assume that any major advertising agency's ads that contain vehicles with tags are fictional but could be found somewhere in real life also. I'd say this would be without intention. License plates add to the realism of an image and I doubt courts could sue over fictional license plates.

I'm not an intellectual property lawyer.