> 1. The dreaded helium leakage. It might not be a problem at all. Think of a party balloon of the foil type. It stays afloat for weeks. And it has positive pressure. A neutral pressure one would leak much less. But maybe not to zero. Ok, now use a double layer. Just like there are double hull submarines, this would be a double foil balloon. The distance between the outer foil and the inner foil would be only 1cm or so, so 99.9999% of the helium would be inside the inner foil. The gas between the two foils would in time become mixed with air, but the amount of double leakage would be negligible. The weight of the double foil would also be negligible compared to the overall weight of the structure.
I assume someone else can address this better, but given how expensive and limited helium is, I don't think we can just write off losses by comparing airships to party balloons. Foil party balloons look noticeably less inflated after a week or two. I have no idea how much gas is being lost, but that seems much more than a trivial amount to the point where I don't think a second envelope is going to help you much. It will have the same outgassing problems as the inner envelope, but will add additional weight and air resistance, both of which will reduce the maximum payload. Relatedly: is it cheap to extract the helium trapped in the outer envelope?
Given the quantity of helium needed for an airship fleet, is topping them up regularly even an option? I'd assume it would need dramatically more helium than is currently being produced, and there's only so much helium available to us without using something like hydrogen fusion.
In college I briefly worked at Brookhaven National Laboratory on an experiment with the Relativistic Heavy Ion collider. That collider has a set of giant collection tanks next to it so that (at least as it was explained to me) in the event of a superconductor quench event, they can try to shunt all the remaining liquid helium coolant into storage in order to limit how much of the valuable resource they lose. I imagine the amount of helium they're using would be peanuts compared to the amount required for a cargo fleet.