Sorry about your fish losses.

I think I would postpone any beneficial pond chemistry changes until after your fish are out of a stressful situation. I don't know if the phosphorous binder and beneficial bacteria cause any rapid changes that might be tougher for the smaller fish to tolerate during a low water level, low oxygen period? (Perhaps our management experts can expand on that thought?)

If you were running your bottom aerators before, then going to 24/7 should be beneficial.

I am not sure about the fountain. Water sprayed into the air should come back down with more oxygen. However, it will also increase the evaporation rate of the precious remaining bit of water in your pond.

My best recommendation right now would be to perform a successful rain dance. Sure wish I could give out some expert advice on THAT topic.