I can't imagine they are actually going to pre-clear at SNN during the fueling stop. In addition to the logistical problems already described... emptying the cargo holds, claiming luggage, inspecting luggage, sweeping the aircraft interior, loading luggage, etc... what happens to the hapless 1 or 2 folks on each flight that CBP decides to give the 3rd degree to?

You would then have the dilemma of either abandoning these folks in SNN or holding the aircraft on the ground to give them a fighting chance of reboarding after the interrogation.

I suspect that the SNN stop will be refueling only.