I know there is a speed control for this, but it is frozen. I am tempted to remove the knob and put a vice grip on the spindle. If not too aggressive, is this acceptable, or am I going to compound my problem?
BAD IDEA. What you'll do is destroy either the linkage or the needle valve that meters the hydraulic oil flow.
What you need to do is remove whatever components (seat or seat base or access cover (some models like mine have an access plate that allows you access to the linkage behind the knob) and lubricate the linkage or rod and joints as well as the valve with penetrating oil to free it up. In a worse case scenario you will have to remove the needle valve from the lift assembly and free it up.
BUT UNDER NO CIRCUMSTANCES APPLY UNDUE FORCE TO THE KNOB OR THE SHAFT.
All you will accomplish is destroying it and then have to shell out the bucks for a new one.
They (shaft and needle valve) need to be lubricated regularly because all the junk and moisture collects up there and renders the valve and linkage un-moveable.