Frp- Mx5100 tranny and hydraulic problems

Recently acquired Mx5100 with only 370 hours.
1. have very loud 2nd trans gear noise above 1500 RPM
2. cannot move the 3 point speed adjustment at all

I have tried to turn that knob just about every time every time I sat in the tractor. All to no avail. I took the advice of one responder and loosened the two bolts, but it seemed like there was some type of spring behind the fitting. Not having a parts manual and not being knowledgeable about how it is assembled, I tightened the bolts and looked for a different way.
The control knob is affixed to a 6" tube that fits over a solid shaft and is held in place with a cotter key. The hole is approximately the same diameter as an 8 x 24 bolt. I went to an auto supply and got a punch with the shaft the same diameter. After I took the cotter key out, I used the 6" punch in the hole with the extra leverage of the punch to turn the shaft. It Worked!
I will turn the knob as a part of my starting procedure now.

I am unsure about the service that this tractor has had. I changed the oil, oil filter, and fuel filter as soon as I got it back to the barn.
I added about a quart of hydraulic fluid and the fluid is steady at 3/4 in the sight glass.
Grease was added to all of the zerk fittings that I could find.
What could a dealer do beyond that other than charge me an arm and a leg?