Hello, I hope the following is of assistance.
You are correct in the fact that the linkage is seizing up on the pivot bush. The bush is plastic and you need to remove the mechanism to clean it correctly.
First remove the plastic cover from the front of the gearbox as shown below.
You can then see the linkage where the cables connect, where the long linkage pivots remove the small plastic cover then the circlip, disconnect the gear shift cables and pull the linkage off the pivot shaft while guiding the small plastic bush on the end out of the other linkage.
Once the lever is off clean the plastic bush using a thin wire brush or as I do drill the bush so it's 1mm wider in diameter. Clean the pivot shaft with a wire brush, this will unscrew from the transmission housing to make it easier.
Once all is clean reassemble using some grease on the plastic bush and pivot shaft.
I hope this helps, let me know if I can assist further and I'll answer as soon as I'm able to.