What is the best workflow solution for editing a movie in FCPX and creating specific effects for it in Motion 5?
There's no really good workflow, I think, yet. The best way I found is to export the video out of FCPX and import it into Motion. Apple thinks that you don't need to work in Motion solely, you only go there to create an effect, title, whatever and then put that thing right on top of a clip in Final Cut. 

The problem there is that sometimes you'd rather want to have all the editing options Motion offers and just want to see how the video's going to look like. That problem is, to my knowledge, not solved yet. I hope this is all going to change in the future.