You are correct that the Mega Exo BB was made to fit a 73mm rear shell but our Speedster had 68mm shell and uses a spacer to take up the slack. The BB was made to work with a 73mm Santana as well as a Commotion.

Our bearings lasted about 6,000 miles. We sold the bike at about 11,000. Bearing life has quite a few factors. BB shell facing if it is off could be killing your bearings since external BBs, rely on the shell being faced correctly. That being said I think Comotion usually does a good job on that. If you have a shop that can do it I would consider it. Some have gone to ceramic bearings to extend the life. Might be a good idea if you ride on wet roads a lot. Personally I think external BBs bearings might be a little on the small side if you want long life. Small bearings are lighter however. Big bearings are heavy.

I am interested to see what others with more miles on a Comotion say.