The 24 spokes on 28-inch rims don’t go of my mind. With the 310 lbs. team (congrats, a very fit team), probably 40 lbs. for the bike and another 50 lbs. for the gear on overnight tours everything could easily sum up to 400 lbs. On hilly terrain, surely not always asphalt under the tires…
Even with a well-made wheel, taking the rims given I personally would be afraid of putting some overload to the system. Assuming the front wheel has been made under the same conditions: The problems have been occurred to the rear wheel. My feeling is, that this wheel is probably exceeding a limit being not able taking the resulting loads on the long run.
Great move, that Spinergy helped with the replacement with no hassle. Nothing wrong with building a new wheel with those rims either. However, I would have a close look on them and probably rethink the whole (wheel-) setup in the future…