@Badwolf, it sounds like you found your main culprit thanks to mango. But I wouldn't doubt there are other things going on here too. I had a very similar problem just this past week. I'm also in a coco mix for this particular grow and the girls are preferring daily watering. What happened to me, though, was that I was brought to my knees because my pH pen stopped accurately reading and I didn't realize it until damage was done...the same damage you have. The pen was reading approx 0.6 too high causing toxicity and lockout due to being below the low end of desired pH range.
Do you have more than one way of measuring pH?
I'm lucky to say that I've not had pH problems in previous grows. Seemingly, my pH meters and pens were all accurate (or at least accurate enough). Right now though, I'm unbelievably frustrated because I have 4 different ways of measuring pH and they're all reading differently. Pain in the ass.
For what it's worth, I'm seeing improvement after a good flush and replace with light nutes and a pH that I *think* is bang on 5.8 (who the F knows though?).