You should never see any kind of "slime" in any part of the fuel system. To me, that may be one thing I have yet to see addressed and that is algae. There is a form of algae that forms and spreads in diesel fuels. Good thing is there is a fuel additive for that, which will kill the algae and it will basically fall to the bottom of the tank or get caught in the filters.
If you see any of this slime in the tank, then I'm willing to bet that it may be your culprit, but if this stuff in throughout the fuel system, it will cause issues until it's gone and I highly recommend adding that fuel additive for the algae anytime the truck sits for a while to prevent it from forming again.
I know you said you added all the other cleaners and such, but did you add any water removers or alcohol to the tank and stir it? If not, I'd still recommend doing that and get the truck started and let it run for 30 mins to an hour a few times a week. This will get those cleaners circulating and work on cleaning the system. I'd also drain the primary filter of any water each time before and after you run the truck.
Have you changed the air filter or cleaned the current one out well? Sometimes those little gauges inside the cab aren't always accurate. Pop that air filter out and start it up and if it idles fine, that was your issue or a combination along with old and dirty fuel/fuel filters.
I'm just trying to suggest all the easy and cheap solutions first before assuming it may be the IP, but I would wait on all the other things that need fixed until you get it to run right. If it doesn't run right and you feel the IP is too much of a hassle to work on and wish to sell the truck, then you aren't investing more time and money into it then you really want too. If that's the case, then maybe in the future, if you wish to purchase another deuce again, I'd look into one that has been well maintained. My first one sat for a long while and needed tons of work and was a money pit and that was before I blew the head gasket, lol. My second one has been a very well maintained truck that didn't sit as much, but was driven a lot. Sometimes those first impressions can just give someone a negative opinion about the truck, but once you develop that positive opinion about it, maintenance issues and repairs are not so much of an effort as it is a priority because you can't wait to drive it again.