I'm trying to picture a 5 HP pump in a 4" casing. It must be a damn close fit. My 4" pump is 3 3/4" dia. which would leave a mere 1/8" all the way around in a 4" casing.
I wonder how many GPM the pump is to use 2" drop pipe or is 2" used for strength, not for flow? Using an area calculation, a 4" casing would have an area of 12.566370614359 sq. in. A 3 3/34" pump would displace 11.044661672777 sq. in. leaving only 1.521708941582 sq. in. for flow past the motor, far less than what the 2" drop pipe has. The velocity that narrow space creates should slow the development of encrustation. However, I have heard of heavy encrustations at the surface of the static water table and pumps having difficulty getting past it.
When I pulled my pump out of my 6" casing it was heavily encrusted, up to 1/2" thick in places. I can imagine there could be encrustations on the inside of the casing too. Around here, drillers use mostly 6" casing.
Anyway... that is a digression. I wonder how hard one could pound downward on 2" PVC in the hopes of dislodging the pump? If it did finally dislodge, it could plummet down out of reach if precautions were not taken to prevent that from happening.
Also, back to the theory of lightning as the culprit. Since the downpipe is a non-conductor as too is the casing, the energy of the lightning hit would all have to be carried by the water in the pipe and the wires to the motor. That said, there should be some serious evidence or lightning damage at the surface.