As of now, the design of Helryx’s mask not yet been determined; only the character’s overall appearance (body) has been. DSS is free to use his own custom mask design in whatever piece of art he would like to create and resubmit.
As far as the mace and shield are concerned, it would be for the best if they followed the design of the winning creation rather than his custom pieces. If the winning model had used their own custom 3D modeled weapon and shield, he would be allowed to substitute his own custom designs. Since the winning model used weapons built out of existing LEGO pieces, however, he isn’t able to change them out for the art portion. I hope that makes sense.
You are correct in your interpretation; he needs to draw/illustrate the Helryx body combined with his mask as if it was from a comic, or just an art piece in general. There are many different styles people use, ranging from sketches, to colored drawings, to digital artwork. You can use other entries as a framework guide.
Both of your interpretations about creative liberties are correct; however, things such as adding pieces to the neck/shoulder area as your son did in the Stud.io artwork entry are too significant of changes. For an example of what creative liberties are allowed, take a look at Galva’s entry that I linked in my previous reply, or for an example that references the more comic-like approach you’re speaking of, look at the following three entry links:
As you can see, these pieces of artwork don’t noticeably alter the structure of the winning body in any significant way, but they still allow for a plethora of artistic expression and creative liberties as far as alterations go. Your son is encouraged to use those entries as well as the other submissions as a point of inspiration for how to stay true to the winning model while still drawing parts differently to express his own interpretation of the character. Alternatively, in the future, if he wishes to create his art in the form of a 3D model like his custom mask and weapon designs, he can do that as well, and once again, you can use Galva’s entry (linked in my previous post) as an idea as to how you stay accurate to the base model while still being creative with the design of the pieces.
If your son would like to submit a corrected and revised entry, he is more than welcome to! The entry period does, however, close at 11:59 PM EDT.