Max was a conduit for the Creation Engine, which was Wildstorm's version of the Infinity Gauntlet. Whether or not he was omniversal or multiversal might be up for some debate, because his stories focused squarely on the WS universe. However, the casual way in which FAR lesser beings traveled between universes and could affect the multiverse in WS suggests that Max would have easily been able to do so. I don't know of specific evidence, though, as to whether or not his powers carried over into new universes.
Based on average showings, Max destroys. By top feats and/or potential, it's likely a bit closer.
It was, yes. But Max also never really f*cked with it in the same way that, say, the Authority did when they'd universe-hop regularly. I think the implied - possibly stated - scope of the Creation Engine was multiversal, but I'd have to dig through a lot of older stuff to confirm. Outside the Authority I was more casual than hardcore WS fan, so I don't have it committed to memory.