Those are all good questions! And the answers are highly dependent on technology. Even with today's materials, we can do things that were unimaginable on the 20s and 30s, and with carbon nano-tubes and the like, the... uh... err... sky's the limit? Sorry about that
With 1920s-30s technology, the answers would be as follows:
1) Pumps and storage tanks to conserve hydrogen: It's an attractive idea, but even today, with high-strength composites, there's no way these could be made light enough to be of any use. And the dangers are alarming. Even an ordinary welding tank, pressurized to 2000 PSI, is a potential rocket engine/bomb. Lightweight ultra-high-pressure tanks frighten me. Which is why I don't use a high-pressure tank for the oxygen system on my hang glider.
2) Aerodynamic forces: You're right, these are quite significant. At cruising speed, even a small airship could easily generate several tons of lift and drag. This limits the speed of something like the Goodyear blimp to less than 40-50- MPH. At higher speeds, the nose caves in, to the consternation of company executives and the amusement of competing tire manufacturers. But the R-505 and its contemporaries are rigid airships, with rigid frames, to which the envelope is laced just as it would be on an old-style airplane. This is good for amazingly high speeds -- well above 400 MPH on the control surfaces of some WW-II aircraft. In our world, the R-100 did have a problem with ripples in the envelope at speeds above 70 MPH our so, but I've assume that with a smaller ship and the benefit of experience, the designers of the R-505 were able to solve this problem. In our world, the fastest ships were the big American flying aircraft carriers, the Akron
and the Macon
, which had top speeds around 86 MPH. The R-505, smaller but with better engine and propeller technology, seems about that fast.
3) Hydrogen generation: This is way complicated. Electrolysis was prohibitively expensive in the early 20th century so most industrial plants used schemes that involved acids and/or iron and steam. I've been slipping details into the story bit by bit, but there's a limit to how much I can say at any one time lest I alienate everyone but the chemical engineers. With the technology of the day, a plant would weigh more than the lift of the hydrogen it could generate. Could advanced technology provide ways around this? Perhaps, but this is a matter for an entirely different story that has yet to see the light of day
4) Diesel-electric ships. This is a way neat idea! And if technology took the right path, it could be a practical alternative to the weight and complexity of variable-pitch propellers. But the fuel still gets consumed, whether diesels drive propellers directly or via a generator/motor combination, leaving the ship with a load of water in the place of what was once a load of hydrocarbons. The great passenger ships of the 30s routinely flew up to a week at cruising speed. With weather routing, taking advantage of storm systems, and shutting down all but one or two engines, they might in principle have stayed aloft for a month at a time and flown around the world without refueling. But there was no commercial or military reason for anyone to try this.
How has the Cruiser managed to stay out of sight? Everett has been wondering about this ever since the attack. In a world without radar and GPS, where bases could be hidden on remote islands and airships could also refuel and resupply at sea, the possibilities are many. But keep your eyes open, for there may be some clues in the near future!