Booya: first of all i feel your pain and the same exact thing happend to me about 6 months ago. I added a powerhead to my macro algae culture tank and two days later my lone molly was gone. If he had been a bird I could say I fed my macros shredded tweet.
First, for the powerhead I added one of those sponge prefilters to the intake so things like fish could not get sucked in.
Second, try a plain old black or siver lyretail molly. Just take about 2 hours to acclimate him slowly. At least that way you will have a live fish in your tank. And do not -do not- feed him for a week. Let him graze off your hair algae during that time.
Finally and most importantly, all of your concerns you have posted, with the exception of the fish in the powerhead, would not have happend if you had established a thriving marco algae marine plant growth first and then done the rest. There would have been no algae on your live rocks, no ph drop, and the tank would be fish ready now. Actually, I probably is fish ready just the power head got in the way. So get all the macros and plants you can get your hands on and add them.
And again I feel your loss. Been there myself. Hopefully in six months this will all be a distant memory.
