I'm kinda..... um...... confused I guess.
I understand hydro linkage freezing up. BTDT. But not on a machine that I use. Only on machines that have sat outside for long periods of time. Has this thing been sitting? ....... Outside? I don't see how else everything would have rusted stuck unless it was.
When you mentioned that the auxiliary hydraulics worked, that threw me off too. They have nothing to do with the hydrostat linkage on the dash. At least not outside of the fact that they share the charge pump. Which, again, has nothing to do with linkage. You sure that the friction adjustment didn't just tighten itself? Or perhaps got a little "rusty" and made it tight? FWIW, I lubricate the hydro and hydraulic levers at minimum annually.