This heavier than air thing. as the guys above say why ?
The only real reason I can see is may be the air at 100,000 feet is so thin that Helium is not much lighter than the air, so no advantage.
I don't know, but a thought, would a strut made of 'material' be it Kevlar or what I don't know, pumped with Helium, to make it 'stiff' be lighter than a strut made of only Kevlar.
After all, you don't need to make the thing lighter than air, your just trying to save weight, every gram counts.
If you start off heavier than air, you don't need to worry about ballast et all, your just making the structure lighter.