I was hoping a real diesel person would be weighing in with more here. But have you actually checked your valve clearances, as someone else suggested? All you need is the right thickness feeler gauge, a spanner, and a screwdriver. I think all does usually look beautiful under the valve cover, but that doesn't tell you if they're adjusted correctly.
Not even close to being an expert on anything but do have some years working on gas and diesel engines. The crank being hard to turn when only the #1 valve train operates is interesting, perhaps an out of tolerance camshaft, bent valve stem, seized valve guide, galled cam lobe or rocker arm? For starters a compression test on all cylinders should be done.
When things settle down when operating the de- compression lever on #1 leads me to think of the above valve issue(s) or perhaps a blown head gasket in the #1 area. Let us know what you find.