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?

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.

Raoul