Seats move around and are replaced all the time. This is how legroom keeps shrinking.
No, the FAA doesn't regulate seating layouts.
Weight and balance have to be met, and exit time is verified.
Since people are heavier today, and carrying unweighed baggage into the cabin, weight and balance are actually not being met.
They're blocking off the last 2 rows on the A320neo because of this: https://www.forbes.com/sites/heatherfarmbrough/2019/09/19/wh...
I'm a commercially-rated airplane pilot. I tolerate the pedantic HN comments, but please realize that I know what I'm talking about, and if you're not a pilot, you don't.