The use of high pressure O2 (16 psi IIRC) was to keep dust and moisture out of the capsule during ground operations
During flight cabin pressure would have been bled down to
3.5 psi for mission
Yes, I think the goal was that if there was any leakage it would be in the outward direction. I always remember those air-to-ground calls during ascent about “cabin pressure dropping”. The first time I heard it I was worried there was a problem but only later learned it was normal procedure.