Need help with a problem. Precipitate on bottom of pool

[Poconos]

This will at least bump the post to the top. Can't answer your all of your question except that shutting off the pump with the heater on should do no damage at all. With gas heaters it is routine to control the pump and let the heater do its thing. Heaters have overtemp and flow switches to kill the gas as soon as flow stops. If flow switch fails then overtemp safety should kick in. Also, the heater should be downstream of the filter so there should be no interaction.
Al