I would say it's a physical impossibility to run more than the three effects at once.  The way the M9 is setup essentially is that there are three DSP "engines" - one for each slot.  And each of those can only run one effect at a time.  There might be some effects that they have the processing power to run more than one instance of at time, but I'm sure there are others that would max it out.  So, realistically, I don't see there ever being a way to change the limitation of three simultaneous effects at once.  If it could be done with the current hardware, I'm sure they would've made it so it worked that way.

Theoretically, any MIDI to USB converter should work, and some people have had luck with the el-cheapo versions like that.  Personally, I'd rather go with a brand name device simply because the thing that's always the potential issue with these adapters is the driver support.  Personally, I trust a company like M-Audio which has been in the interface business for quite a while to know how to create working drivers over some no-name company.