The slider gets stiff on the right-hand side when near the closed position. It will only align up closed when enough force is applied; and it springs slightly open (but it still requires force when opening, during the first inch of travel).

I removed 4 screws to separate the screen. Sliding the screen's inside metal plate, I can't see any obstruction. Removing the screen's bezel & plastic surround seemed to be a way to get better access to the slide's rails.
At first I was trying to pry the plastic off, which started to break. Then I just removed the metal bezel.




Can the remaining plastic surround be removed? (Should it be removed?)
What is a likely cause for the sliding mechanism to get stiff?

(only just purchased the phone, so can only speculate about it's history)

I've not done anything like this so I don't have an answer, but get a service manual and that might help answer your question.
the two I have are called:
Nokia_N900_RX-51_Service_Manual_Service_Level_1_2
Nokia_N900_RX-51_Service_Manual_Service_Level_3_4
Search for them online.