Correct on both counts.
They are limited to one void, obvious legality trumps the interaction of the techniques. The path has shiba bushi as a pre-req so if it states only one void then that is obviously developer intent.
They still only get the listed benefit, the rank 5 tech doesn't double the benefit as you cannot spend 2 void on the tech.
How the rank 5 Shiba tech works is still debated. By the rules you'd have to be able to spend 2 void on an effect to get the benefit by only spending one. However, Shawn has made a ruling that basically just said that the Rank 5 tech doubles the effect of spending void.