I bought my Troy Bilt pony at Lowe’s new in 2016. We had the same problem, starting about a month after we bought it. The first time it happened, I had to move it out of the way, so I shifted in in neutral and pushed it. When I went to use it, the lever was in forward but it wouldn’t move. Of course Lowe’s wanted nothing to do with it unless I paid a hefty fee for them to fix it at some repair facility of their choosing. The warranty wasn’t worth crap. Like you, I cleaned the pin and manually put it in all the gears to make sure the transmission was functioning properly. It has worked pretty well for the last couple years, as long as no one shifts the gear selector while the mower is turned off. Now, I have replaced the blades and both drive belts, but the trans is locked like yours. Any suggestions?