Electronic – MOSFET not keeping charge – Help understanding

I think you've got it mixed up. The body diode is a part of the structure of the MOSFET and does not help when switching an inductive load with a single MOSFET.

The zener in the symbol represents the avalanche rating of the MOSFET and is of use when switching an unclamped inductive load. You won't see it conduct unless you exceed the voltage rating \$V_{DS}\$ and get to \$V_{(BR)DSS}\$.

If you are actually getting +/-12V out of the pulse transformer, I don't see any obvious reason why that part would not work. Your negative swing out of the pulse transformer does have to be sufficiently large to turn the control MOSFET on, when loaded with both gate charges (-10V would be good).

A pretty normal explanation for this is as follows: -

You said the first MOSFET (on its own) got hot but continued working and my 1st observation is that you may not have been driving the gate "hard" enough. This would have caused some switching losses and possibly some conduction losses causing the device to get hot but it still worked.

So then you put another device in parallel and in doing so you have "shared" the gate drive current - the gate capacitance has effectively doubled and the rise time and fall time of the gate signal has doubled increasing switching losses for the device that begins to turn on first (not all FETs are identical). It rapidly overheats, melts and short circuits.

If you want this confirming, produce a circuit diagram of what you actually did.