With cloning, you don't want to feed the foliage, you want the formation of roots. Ditch the Superthrive, keep the enviornment and soil humid, (humidity dome or cover) do not feed the unrooted clones ANYTHING except properly ph'd water, and do not spray the leaves...they have a tendency to mold or rot.

The leaf yellowing (and die-off) is the plant realizing it need the nutrients, and she's scavenging those nutrients from the leaves. This is absolutely normal. She is using those nutrients to produce the roots. If you feed the leaves, the roots are treated like an unnecessary bother, and may not form.

MG soil is a tad tricky. But MG isn't a mega-nutrient rich soil. It's a slow steady release, and I've cloned many, many plants in it.

Do not blast with full-on lighting. Raise the lights, which lowers the heat. Again...you are trying to grow the underground stuff, not the above-ground stuff. Just enough light to keep her (them) alive.