Originally Posted By: paul

in the diagram I posted with the block and incline the incline
is showing as a force , but not the force that the block would
exert in the opposite direction.


That diagram would be used to analyse the behaviour of the block, not the incline. So it includes all the forces on the block, and none of the forces on the incline.

If you wanted to analyze the incline, you'd do a separate FBD for that.

If you wanted to analyze the complete system, you'd still have 2 seperate FBDs, but you'd find that some forces have the same magnitude in both diagrams, so you can connect them together with equations to describe the complete system thoroughly.

Come on, do FBDs for the components of your mechanism. Do each moving part with a seperate diagram. The diagrams you've shown are incomplete and ambiguous. FBDs are crystal clear because they're done with such simple rules.