Yeah, these S771 holes are very big, but if you apply the hex plate it should narrow them enough to just stick the EK mounting screws through. Doesn't matter if they're weak, normally the Ek Supreme is mounted without any backplate, so you'll be fine.

And no you can't use the rectangle ones with the EK Supreme, obviously, unless you drill some holes. But as I said, backplate isn't really needed for these coolers. Also don't tighten them too much, they don't cool better but sooner or later the mobo will bend.