Looking at the pics, I'm pretty sure it's kanekalon.
Yes, fabric softener works very well.
You don't want to "dunk" the head in boiling water for this project, because the resulting hairstyle won't be very pretty.
Boil a pot of water, and add some fabric softener to it.
Stand the doll up in a sink, bend her arm out and away from her body as much as possible, and hold her hand! You're not holding her hand to comfort her, LOL, you're just doing it so that she can remain standing upright.
Now, pour the boiling water over her head. You want to be careful not to burn your fingers as the water gets poured, so that's why I said to bend her arm outward from her body when you hold her hand.
After pouring the boiling water, comb her hair with a fine-toothed comb.
Then, pour more boiling water over her head again, and comb again.
After that, pour ice water over her head, and comb it again.
Then, let her hair air dry, and the following day, you should be very pleased with the result ;)