Do that tomorrow afternoon. I was able to spray some boeshield on the linkage and it seemed to make a improvement. Could there be a bushing issue?
It could be. When you try shifting the lever manually, are you turning the prop by hand? You should especially when trying for reverse.
For this test, the shift cable linkage has to be disconnected.
Hopefully, shifting works in that test.
If it does, make a note or a mark as to where the neutral position of the lever is as well as the lever position for full forward and reverse.
Now with the gear box in neutral, see if the cable linkage with the shift handle in neutral, lines up with the neutral position of the shift rod lever. If it does not, adjust the cable accordingly. Now reconnect the shift linkage and try to go into forward at the helm. You may need to rotate the prop.Verify that the lever moves as far as noted earlier.
If that works, try the same for reverse. You probably will need to rotate the prop.
While you are at it, check the linkage where the sheath of the shift cable is attached to the motor castings. You are checking the part of the cable that shouldn't move when the shift handle is moved. Sometimes that connection pops open or breaks and the cable sheath moves instead of the cable core wire.
Let us know how it goes.