you're actually making it way more complicated, than it needs to be, just have a larger sprite (being 4 tiles in side over 1) for the boulder, and make a second alteration of the strength tm code, i know you dont script but you will have to atleast copy and edit one. basically want to make the move route not be one tile when pushed but two, and then would have to have another invisible event (or more) to block the player walking around, that are removed when the strength rock is pushed in. i've not looked into strength myself, but it will be hard to do this without atleasting looking at the base strenght script, or how base strength rocks work.