The tar on the valve stems is probably from burning old fuel. Just happened to me on my generator. Caused sticky, slow closing valves and two bent pushrods. Most of it probably formed on your trial run. Sounds like the same scenario. Runs OK for awhile using old gas, then shutdown for awhile, then runs like crap with valve issues on next startup. I had black tar on all the valve stems that I eventually cleaned out with with a spray power tune product.

Once you had phase seperation in the tank it's probably difficult to get all the water out without vacuuming out every corner of the tank and cleaning it. I just had to replace my aluminum take on my Checkmate this year. Corroded as hell and it was impossible to suck all the fuel out of it while it was still in the boat.