OP here. I took the lens out again today, gave it another chance to see if it really is faulty, and my conclusion is that it obviously is. Took two pictures, one at f/3.5 and one at f/11, focus double-checked with split prism, and they still look very fuzzy. I deleted the one at f/3.5, it was that bad. I will attach the other one, I'm pretty sure there must be something optically wrong with the lens, so I dismantled it. The problem is that I don't know what could possibly be wrong with it

it has no fungus, signs of separation or dust inside, but can the optical elements be misaligned? I see that some screws had been previously forced, so obviously somebody else played with it before. I don't have a lens spanner at the moment, but I will acquire one if there is something I can do to save the lens. Thanks!