"making right click spawn a temporary player character" this would be perfect. I think letting you playtest as you edit without having to press any other button is really good and making that feature as user friendly as possible should be an end-goal of sort.
Also just for clarification, is the diamond a separate player entity like what you had in your game? For some reason I was thinking it was some sort of 'interact' tool which let you do more advanced things, but I guess not. Since you can't really have the platformer player and the diamond character at the same time, it would make sense to have it as a toggleable button that lets you switch the player from one type to the other instead of two separate buttons. That way users will understand what the diamond is immediately and not get confused like me and others have been.