Turn off the breaker serving the heater. Use any multi-tester you can - they are cheap. You want to measure ohms or resistence thru the element. Disconnect the wires from the element. Simply put one lead on each wire connection. Not the wire. If you read resistance the element is probably ok. If...
The last time I had to subsurface support a piping system we were to assume the soil will fall away in time. This of course, would leave the piping "hanging". Your only concern would be the material of the supports must be resistant to acidic soils and moisture. Galvanized or...
I would guess the venting system is either too small or blocked. Air pressure in the system is increased by simple displacement thereby moving the liquid in the adjacent bowl.
It sounds as though you have hot water migrating into the cold water due to thermal expansion. This condition is prohibited in most codes these days, but it wasn't addressed until recently. What we do here is install a check valve in the cold water supply to the heater and we add an expansion...