Hi J,
I was just reviewing the install process for the misty paint for myself. It does contain all files necessary for the folder. I suggest you delete the files in your texture.misty folder and copy/paste the ones from the zipfile in there. You can't tell what paint scheme it is by the names. They are all the same filenames. So, yes. You shouldn't need to reinstall the entire plane to fix the misty paint. (on second thought, with that error it might be a good idea to
start over with a fresh install of it.) I doubt the auto-installer would delete anything, but may have trouble rewriting files, however unlikely. Best to do fresh install.
I don't like floatplanes (no wheels), so I probably have never clicked on that one. There are model versions for the ski and floatplane that have lower windows in the doors (model.DW, model.bush), for better visibility.
Edit: I am flying the floatplane with door windows in P3D right now. So mine is working, "out of the box".
Liveries are defined in the aircraft.cfg files, and the presence of corresponding texture folders. Backup your aircraft.cfg files and texture folders of anything you're worried about losing before doing anything. (unless you've got copies already in a zip file somewhere.) But all the liveries come in the install, except the Misty repaints, which you can get here. So, no worries mate.
I also notice an SP1C185FFSX.exe file in the install folder. That sounds like an update. I don't remember, though. It would also be good if you installed the B. Stolle flight dynamics upgrade too. Then apply the misty repaints after that.