I was curious about your build and finally found it. I am even more impressed now with the whole thing. My only addition would be from the "belt and suspenders department". From what I can gather, the piece of square tubing on the side of the channel is what transfer any loads to the FEL pin or frame in the forward, aft and outboard directions. However, as you rotate the swivel seat inboard towards the operator seat with your weight, now you have a moment load (torque) trying to move the top of channel away from the end of the pin and only the "U" bolts at the bottom would be preventing such thing from happening.
My suggestion would be that since you already have a hole on the square tube piece to verify pin engagement, enlarge the hole, weld a nut, put a bolt in there to capture the tip of the FEL pin. At that point the FEL pin would also take any inboard loads from the lift. I might be wrong, but if my assumptions are right, this might add a little extra safety. I hope this makes sense.