I used the heavy duty II system for 7 months.
The only thing i changed was the days between workouts. I trained every monday and thursday.
Now i've started the "old" heavy duty training m/w/f.
I don't think i'll ever go back to multiple sets.
I follow the routine listed on the site you have found, but i have added db rows after deadlifts and shoulder press on shoulders/arms day.
I think they have made a mistake calling it a pre-exhaust set. What they meen is superset at which you pre exhaust the muscle doing the isolation movement before the compound. This way you will not stop the compound movement when your weakest link gives out, but only once you have totaly exhausted (failure) the muscle being worked.
doing flys or pec dec to failure superseted with incline presses to failure, will ensure that you have totaly fried your chest, and you will not stop because your tris failed.
Increasing the weight is no different with HD. Once you get to the desired reps for an exercise you increase the weight.
Sometimes i increase the weight only for the first exercise of a superset, sometimes only for the second and rarely for both at the same time.
The only way to know that you got stronger is if you lift more weight, or if you lift the same weight for more reps.
Sometimes with supersets, when i increase the weight for the first exercise, the reps for the second one go down by a rep or two, but the next time i do the same workout they go back up again.
If i was you, i'd start a training log and try the workout for a month and see what happens.
The frequency is something you need to work a litle and see what works best for you.
Usually i do legs on monday and on thursday i do either chest/back or shoulders/arms.
Its almost 1 in the morning over here, i'll check back again tomorow in case i missed something.
I don't know what you mean by standard HIT, but i haven't used any other hit routine. I've been doing this routine for almost 2 years now, and i've never hit a plateau for more than 2 weeks.
None of the exercises i added are supersets.
As you know, the rutine is extremely low volume and i wanted to make sure that i hit the muscles enough and in all possible ways. That's why i added the 2 sets.
My training has been very much influenced by Mr Mentzer. I have used his system to the tee with very good results and now just incorporate many of his overall theories, along with my own and a few others' that I respect, into my current program.
Mentzer was a very very important contributer to training theory.