Is the cassette the same width as current 9 speed cassettes? In other words, will a 10 speed cassette fit my existing rear wheel hub and frame?

I think so…. but maybe wrong.

helpful, i know.

Shimano 10 speed road cassettes are 0.75mm narrower than 9 speed, so they fit with a spacer. SRAM 10 speed cassettes don't need a spacer, it remains to be seen whether 10 speed Shimano MTB ones will.

The basic answer is yes, it'll fit.

Yeah I'd imagine so, they'll want as much take up as possible, so making it fit existing hubs would make sense…