I wouldn't worry about the 9V / 9.6V thing. It's an insignificant issue, as a lot of 9V adapters are marked as 9.6V.
As the Echo 600 is a fairly simple analog delay pedal, I'm 99% sure it wouldn't need anywhere near as much as 100mA. It'll work fine on any of the outputs. Regarding a higher voltage... I don't know if the pedal will support a higher voltage, as that'll depend on the overall design and component values. If it can, you'll have the benefit of some extra clean headroom. If it can't, you'd fry the pedal... so I wouldn't experiment with voltage until you are certain it can work on 18V.
I'd ask Malekko about it, since nobody would know better than the maker of the pedal.
