Again it's very likely that Hellstrom's power source, the Darksoul, is 4-D, at that's where other 4-D stuff like his sealing comes from (In fact basically everything he does comes from his internal power, even summoning his trident), and an overload of its energy killed a guy that was threatening Satan (
Scans). Admittedly it is a little inconsistent cause at another time Satan still beats him with the Darksoul albeit with some more effort, but in that run Satan was >>> Mephisto and other Hell Lords so I do think it makes sense for Hellstrom's power source to be 4-D. I think.
So yeah I wouldn't be sure that Harkon powernulls, especially since Reaper said it himself it's not necessarily likely he'll lead with that powernull.
Skillwise he's alright, he's fought hordes of demons by himself and brawled with Satan and other very old demons, he's got a couple of other decent showings but he's far from the most skilled Marvel character, and he does indeed usually lead with fire blasts. But as you said he might just be stuck in a 9-A body if he actually
is powernulled
Hellstrom's regen is somewhat fast for minor injuries since he gets shot in the leg and two panels later he goes "wtf it's not healing", unknown for bigger stuff. I apologize if the negation is a little poorly worded, basically demons regenerate from any injury given to them by mortal weapons, and only stuff like Daimon's own demonic powers or special weapons like the Breathing Gun can deny that.
