How long has it been since you boosted humidity and reduced heat? Also how long since you repotted them?
What did they have for roots when you repotted them?
Exactly how tall were they when you repotted them and buried half of the main stem?

I could probably tell you a little more if you posted some pictures of them but I'll do my best with what you gave me for info and should be pretty close. First of all by the sounds of it you transplanted them too early. They're fragile at that stage, they should at least have a solid set of normal leaves before you introduce any sort of possible disturbance to the roots. In addition to that... unless I'm wrong about this during the seedling stage... you should never bury the main stem above where it naturally came out of the soil. Doing that can and will cause your main stem to rot cutting off the entire vascular system. Now.. if there is any chance of saving them I'm not even sure if you should try to completely unbury it cuz it may not be able to stand up, or maybe unbury it and very gently help it stand up again.

Here are A couple other things that have worked well for me in the rivival of seedlings... My recent problems were caused by fungus gnats eating roots off of them at around the same stage as yours may have been. anyways..

I added some rooting hormones, (same thing I use to dip clones into so they start rooting), in the powder form and sprinkled it on top of the soil around the stem then used a spray bottle to slowly soak it into the soil at the base of the stem where the roots would start from.

I also added a short period of darkness instead of 24 hours of light. Started at 2 hours for a few days then to 4 hours of darkness. I use T8 Florescents and 6500k CFLs though instead of a MH for that.

More humidity would be good too, you can get a cheap humidifier for less than $20 at walmart. Also a very light mist on the leaves, so long as they're strong enough to stand up to that. And remember not to feed them any nutrients until they're a more mature. Not sure if you mean you were able to boost the humidity to 70% in the tent or just in the whole room. If you mean the tent you should definitely increase the whole room, high humidity until late in flowering is best. Lower it when you have some good buds so you don't run the risk of mold.

Let me know how it goes and if you're able to post some pictures that would be great. Oh and as far as humidity tents go, it is really cheap and easy to build one with a few 2x2's or something similar, some clear plastic or another material that holds moisture and doesn't block light, a stapler, some tape and screws.