USERRA does not require that you go. Between the individual and their unit. In my case, if doesn't pass, will not go on military since my unit is flexible.
If MOU passes, still under no obligation since DOD can change their plans. You are under no further obligation (again, USERRA). You can return early as well. The MOU military leave only applies to the extra benefits one receives under the MOU program. Normal military rules apply.
If furloughed, USERRA does not require UPS to provide any benefits not required by stautue. If MOU passes, and subsequent furloughed while on Military duty at time of furlough, EB says you would receive MOU benefits for the duration of Military Leave that you took under the MOU.