If a fully pressurized spacecraft adds strength to the structure, then why weren't the earlier US spacecrafts built that way? To avoid nitrogen containers and their weight?

I'm not certain what spacecraft you're talking about. All of our manned spacecraft were pressurized from Mercury on. If you're talking satellites, then once you get past launch, the structural loads are minimal. The Soviets used pressurization on several of their unmanned satellites because that allows for simplier avionics. Vacuum has a lower electrical resistance than air, so it's easier to arc out in high voltage applications. Americans get around this by developing electronics that work without pressurization. The Soviets got around it by using simplier, less expensive electronics in a pressurized satellite. Both approachs work, but the Soviet technique resulted in heavier satellites.

About the vehicle landing entirely on the Moon, the alternative is a moon-shuttle (moon-orbit to moon-ground). But this means a reusable one and therefore a way to refurbish it with fuel (that must come from somewhere) but that can't happen in the beginning of the programme.

True enough, there's a lot of dead weight landing on the Moon if nothing is left in Lunar Orbit, but that would only be necessary in the beginning. Unless NASA accepts that the newer LM version isn't reusable.

This two vehicle shuttle approach is exactly what I'm advocating. Manned space vehicle development is expensive, so why not do it right from the beginning? One vehicle would be intended to ferry people and equipment between LEO and lunar orbit, then return the people to Earth orbit. The lunar lander would be designed to carry a substancial load to the surface, then return to orbit. I'd like the orbital vehicle to be able to carry a good payload, say 20,000 to 30,000 KG from LEO to lunar orbit. On the first mission, that would be mostly taken up by the lander. On subsequent missions, you could carry supplies like propellant to refuel the lander or equipment for use on the lunar surface. The lander should be able to carry several thousand kilograms to the surface with enough propellant to return to orbit. Eventually, if harvestable ice is found on the moon, it should be possible to refuel the lander on the surface and perhaps carry enough to refuel the orbiter. That would reverse the logistics train for both vehicles and greatly reduce the amount of supplies that must be launched from the Earth. It'd probably take at least a couple years to develop that infrastructure, but it would be useful.