Vehicle in parts, and despondancy? (lol........ oh yeah.)

If you get no luck with the signwriters, you could try ringing 3M technical advice, they can take a while to get back, but if you explain the problem (and if you tell them you think the decals might be printed on the 3M film) they just might come up with something. You'll need to speak to a technician not the people at reception who will try to relay the message, so it will probably take follow up calls.

Otherwise you could maybe try green cutting compound....? if you've got an old tin of it lying around, it might work, maybe, but I dunno this could be hairbrained.
It would probably take a heap of effort, and you could still bugger up or rub through the paint pretty easy, especially with an electric buff, so i'd be testing it on the door or tailgate you were gonna replace first see if it works, and do it by hand. I wouldnt bother buying a tin just to test though, it quite likely wont be abrasive enough. Maybe you could ask the panel beater who took the paint off the door if they have the end of a tin you could borrow to see if it works. I dunno, this could be a bum steer. Bad idea? I wonder what others think?