Either you have a corrosion problem from water entering the BB, or possibly when the set screws were tightened they raised burrs which are jamming the eccentric.

The tip-off as to which is whether you can move the eccentric slightly. If there's zero movement, it's likely corriosion, but if it budges slightly then likely a burr, and some extra force will do it.

I move things like this not by way of a pin spanner, but by holding the largest bar which fit's the pin holes into one, and levering with a small crow bar between the inserted rod and the spindle.

As pointed out above, the desired chain slack is such that there is still visible slack at the tightest crank position. If it's rigth you can still rotate the crank slightly to visibly transfer the slack from the bottom to the top loop and back. Since chainrings are rarely (if ever) perfectly round, make sure that the chain never gets tight as you pedal.

If it is corrosion, you might start by pouring some ammonia down the seat tube, and letting it soak for 24 hours. Once you have it moving, remove it entirely and coat it and the shell with grease before inserting.