I found many years ago that the best technique for removing decals on cars was hot water. The hardest part is directing the water onto the surface. I would boil water and dribble it onto the surface. Once the decal has a loose corner, pull it up as you continue to dribble the hot water on to the joint between the decal and the paint. A guy showed me this a long time ago and it will remove the decal a lot faster. Of course, depending on the size, you might have to get several containers of water to do the job.
It's disappointing to hear that acetone will bother Imron at all. The DuPont Imron I used to shoot on cars/bikes/trucks/airplanes was totally unaffected by acetone or any normal solvent - laq thinner, Mek, etc. Back then there was only one version of Imron, now there seem to be a dozen! I use acetone on our boat paint (Imron- 1992) frequently to clean varnish or whatever spills and it has no effect except to remove the spill.
From what I can understand from several painters that worked in our previous marina, neither current Imron (at least the version they used) or Awlgrip is remotely comparable in durability to the original Imron.
Ghosting will totally depend on how long the old decal was there. We renamed our previous boat and the ghosting never faded away but that was on gelcoat which is known to suck for color longevity. Either Imron or Awlgrip well resist fading much better so it might not be much of a problem.