I have a feeling your blades are loose. Check your blades and make sure they are snug before venturing into the transmission case.
I don't think so. Was there a problem before you changed the belt or is that the reason you changed the belt? When was the last time you replaced the concave washers that tighten the blades to the spindle?
The PTO clutch is wet and not adjustable except for lockup pressure. I suggest you get a workshop manual if you want to set the lockup pressure, it is adjustable.
I have a manual, but don’t see anything about the lockup pressure.