I don't count Mook Jong as IB training. The Mook Jong is just a CMA version of the Heavy Bag, which is sprung to resemble the resistance of an opponent, just like the heavy bag is weighted to resemble the resistance of an opponent. Toughening from the Mook Jong is similar to toughening from the heavy bag, since the aim of the mook jong is learning the grabbing, trapping and punching patterns first and foremost.
As for limbs knocking, it may or may not be useful, but it wouldn't be my foremost training, since I don't have the time for it. Rubbing my forearms together has been helpful, and only takes up a small amount of time, doesn't cause bruises and builds some muscle in a dynamic tension sort of way. I can do this on my own, and use my precious partner time for doing live drills or free practice (sparring).
Plunging your hands into hot gravel is stupid full stop. Markulous - don't do it. Your future wife is just going to love your leathery hands, and your future self is going to be of the same opinion on the subject of the state of your joints. The only point in plunging your hands into hot rocks is that an idiot can do it, and an idiot can teach it.
Forms are good, full stop. My boxing teacher teaches forms (shadow boxing - one two etc.), my judo teacher teaches forms (uchikomi) and if you want to train alone, and most serious martial artists do, it is very helpful. Even my piano teacher taught forms (scales, five finger exercises etc.).
I prefer training where my form is coached for a short time, and then I do lots of sparring. When I get home I practice the forms.
Having a poor understanding of how forms can be applied will make your fighting ability worse than someone with none at all. That would be your teachers fault and your fault, but these things should come out in live sparring, or more cooperative partner work.