timekeeper- Can't really do that because the mass of the roots will be wound up all around the bottom. Don't mess with the rootball. I meant more in the line of lifting the rootball out of its container, filling a little soil in the bottom of that container, and placing the rootball back. Then filling in the sides with more fresh soil. Having a friend would be easier (if a friend already knows about the grow). You will not cause any major harm as long as the roots are undisturbed. This way at least a large portion of the roots will have properly ph balanced soil and should help.

I'm still not 100% positive, because the soil is so old and the ph is so high. Plus you can't really rely on the ph drops for run-off. Also all the different soils you have used makes it even harder to know what's really going on.

Reading back, it could also be that there is a large build-up of nutes in the soil causing the continuous burn. I didn't notice anything about flushing yet. I would maybe try that first, 3 times the amount of the container size (if 3 gal container, flush with 9 gals of properly ph'd water) A drop of non-antibacterial dish soap might help release some of them.

So I'd say try the flush first, wait a few days to see how the new growth is doing, and then use the transplant as a last resort.

Hopefully someone else can step in and confirm or give better advice...