I'm pretty sure you won't be able to program your RMB the way you want, but you can access Custom Properties of an active SW file by clicking on this icon:
I think that's pretty easy. As for the rest of your questions, I'm fairly new to working with Custom Properties myself, but I'll try to help. I think some people assign part custom properties in their title blocks, but I don't because almost all of my drawings reference a combination of assemblies and parts. The custom properties in my title block call out drawing properties like drawn date, drawn by, project number, etc. I use custom properties for parts in notes or balloons attached to drawing views of the parts.
When in a drawing, you can call out part properties in a note by clicking on the "Link to Property" icon in the Text Format section of the Note Property Manager. That will bring up a dialog box. "Current document", which is drawing properties, will be chosen by default, but you can click on "Component to which the annotation is attached", and then click the drop-down to get a list of available properties to use in the note.
There is a lot more about properties, too much to put here, and I'm still learning myself, but hopefully this will help a little bit.