"Cable from lever SEEMS ok" This was confirmed how? You might want to locate and remove the clevis pin on the clutch side (item 6 in the diagram) and see how the lever and cable operate with the pin out. That might help narrow down the problem.
Force it how? It won't move without effort. The fork it's connected to is trying to compress a clutch pressure plate. If you can't move the lever with a LARGE like 18" Crescent wrench or a two foot length of pipe you're going to be splitting the tractor anyway. You won't be harming anything that isn't damaged already.