The shift/throttle lever typically does NOT screw onto its shaft. The shaft has longitudinal ridges that engage similar ridges in the shift lever. This is the lever can be positioned at various angles relative to the shift mechanism when you mount it.
Typically the lever is held on the shaft by a set screw. Also typically, people don't maintain the thing and don't put the set screw in with anti-corrosion grease. So typically, the set screw (usually stainless) gets corroded into the handle (usually alloy). The only fix is to drill out the set screw, pull the handle off its shaft (hopefully it won't also be corroded to the shaft). When you put the unit back together, you'll have to tap the new bigger hole in the handle and put in a larger set screw (I don't use a set screw for this job; I prefer a hex head
bolt with the head standing proud of the handle. Not as "pretty", but sure a lot easier to get loose!).
Re the sticky neutral knob: typically it is also a matter of corrosion
between the knob (often its alloy) and the hollow shaft (usually its stainless). Once you get the shift handle off, you can unbolt the unit (the red screws), take off the shift and throttle cables
, disassemble the unit, clean out any corrosion
, fill it with grease, and put it back on. Sh-t happens and things wear out (faster if they are not periodically maintained). You are lucky the shift mechanism didn't sick in forward or reverse when trying to dock
the boat. This can happen if the internals of the shift mechanism don't get greased and corrosion sets in.