Big Mom is an endurance monster and she can also heal. She can take attacks that pulverize her organs and bones and is relative in endurance and stamina to Kaidou. The 8x oneshot value
represents the difference needed to be able crack the skull of your opponent in a single blow, and while the gap between them is beyond that (9.3x) it's not nearly enough to say that she'd go down in just a few hits.
As for Fa Jin, the difference between the two is still only slightly bigger than the difference between their bases, with Big Mom's innards
actively healing whenever damage is taken. This is without mentioning how Big Mom can continue to massively amp herself as the fight goes on. Just 1 year worth of power is what's listed on her profile
Yeah she has pretty strong observation Haki.
Being able to affect the souls of others through fear is directly superior to anything Shigaraki or AFO have shown.
It 100% is 1:1. There is a large difference between
freezing a crowd of hundreds through fear and straight up
knocking out 100,000. Also I'm not sure where you got the idea where the Fish-Men were weak-willed?