I have to agree about reducing the photoperiod...some planted tank can pull off 10-12 hours per day. Might want to try closer to 8 and see how that works.
what are the results of a recent water test? ammonia, nitrite, nitrate.
your algae doesnt seem to be a problem from stuff being in the substrate it sounds like it is from too long of a photo period with swings in co2 levels.
a 75 gallon tank, 2wpg, co2, canister filter, EI dosing i dont see a need to add or subtract anything. just plant it and let it be. i wouldnt vac that substrate...just 'hover on top of it with the gravel vac' to suck thestuff off the top layer.
Make sure your substrate and filter doesn't become too clogged up with mulm and also make sure you have good water circulation around the whole tank.
It kinda sounds like you know you need to do a heavy cleaning. and with bga you should run the EM treatment as well, after you heavily vac the tank. I'd suggest running a small cannister as a gravel vac, and just go over it for an hour, churning as much of the substrate as possible. Then I'd do a large water change, and start the EM. I'd probably cut the light back a bit as well, but I don't think that's what's causing your problems.
In the long term, you should study your water movement, and try to get more flow down around the substrate, it should help your filter grab more detritus by keeping it from settling.