Looking for input on my problem - pls see pics

My current plan is to transplant to larger pot using the same medium, Promix BX - is that something that should be avoided?

No, not necessarily. Lots of people use it with great success. I personally don't have a lot of experience with soilless mediums. A transplant is in order, I would go with straight water for at least a week (maybe 1/4 strength Cal Mag as you are using Distilled Water). Let them get a little established in their new home and then bump up to 1/8 strength. You tried 1/4 strength and they went "ouch" so try 1/8 strength for a week, then go to a 1/4, then slowly bring it up from there. Generally I don't hit full strength until I'm about 1/2 way through flower.

Again, I don't usually grow in Pro Mix, so maybe someone else here will have some insight into starting strengths, but given your current situation this is the route that I would follow. It's important to also note that your old growth is now damaged, it will not return to its previous glory. When you have the problem solved the plant won't transform through magic back to a healthy looking plant right away. The new growth will come out nice and green and happy and the problems will stop spreading. That's how you know've you have resolved it.

Good luck, and I said, hopefully someone with ProMix experience can lend a little insight.

Ya... What he said...lol

PPM is a huge problem when growing in small containers. at one time i had like 40 32. oz. cups, and im sorry to tell ya without a ppm meter you will continue to have those overfeeding issues. and ya soil is " forgiving " but by the time you see the problem in soil it takes alot longer to fix it, and then you end up adding to much because you dont see results in a day...sometimes 2. GET a ppm meter!!!!!!! your PH is fine.