re: Long duration flight with lower atmospheric pressure
Every manned system I've worked on specified normal Earth atmosphere. We're already tweaking human biology enough with long duration spaceflight. Why change any more variables than you have to?
Costs you some weight, but not all that much. In pressure vessel design, 1 atmosphere is almost lost in the noise once you add safety and structural factors.
Only major downside is the current silly low-pressure spacesuits, requiring extensive preparation to use, but a decent 8 psi hardsuit would fix that.