To answer your mounting question, all kind of imaginative mounting systems were devised to attach a 120mm fan to a HR-03, the most common being a series of cable ties but you can also use fan brackets, elastic bands and have even seen a bolt through system using thin screws, bolts and metal washers.
For your other question, someone else will again have to answer but from what i could read, few people ever consider anything else than a nexus for 92mm fans and i didn't see too much 92mm PWM fans reviewed yet. The best you could do is review the latest 92mm fan roundup at
and see if one of them would be good enough.
As for the PMW wiring question, provided the power/amperage limit is not through the roof with the new fan, i see no problem in making a new wiring but then again someone more knowledgeable about PWM will have to confirm that.