This must be really frustrating, reading your process it sounds like you're doing all the right things. Just from anecdotal reports, I'm convinced that BioBizz is experiencing a major quality control problem with their manufacturing, every other thread in the infirmary is someone who seems to be doing everything right but getting terrible results using either LightMix or AllMix.
If you have some unused potting mix, a good test is to take equal parts water and potting mix (use volume to measure) and slurry them together. Let it sit for a few minutes, then drain off the water and test the pH and EC.
If you can post back those results, it might help to make a diagnosis. I'd also back the light of just a bit, I think it's maybe about 25% too close, maybe try 80cm or more just for now since they're seedlings.