Stuff cleans up well, I would scrub with a brush as Fred suggests, I found LA Totally Awesome worked well as long as I rinsed each section well to remove all the dirty cleaner.
You will need the surface really clean and well rinsed in order to paint because painting over dirt and grime don't work all that well. I'm betting at that point you will be like "why bother doing anything else?"
I would be a little suspicious of water leak having been responsible for the bubbling up. Removing to re-glue involved working from an edge, I used a plastic putty knife for some of the work and a metal putty knife to slice right against the surface when the glue was sticking especially well. Water running down behind ensolite will leave a trail you will be able to spot when you have it lifted.
Suggest working in sections rather than whole big areas. Stuff will contract a bit and is easier to replace if still held up and in place by the other 2/3 of the sheet.
Without foam insulation there will be a tendency for the walls to want to sweat condensation. Although some have stripped the ensolite and just painted I personally would be reluctant. Even more reluctant to try to cut all those compound curves into any other covering.
You need to see it clean to know what you have, and even if painting it is not a wasted effort. Then the caulk in the seams or you can buy the white foam stick tape. Better to go after market than to buy the Scamp
stuff. I used the seam tape from Scamp
and had to add 3M 77 spray adhesive to every piece eventually, it just fell off otherwise.