I'm afraid I'm not much clearer about what you are asking, but I'll try to answer your questions.

Revenue Management are responsible for how many seats are showing available in each fare bucket. A computer makes the primary decisions, though I imagine a team of people will supervise the process. I do not know the ins and outs of this process, save that it relies on historic data about the likely loads for a particular flight and the team (plus computer) try to make their best guess at what they can sell at what price. Obviously, it is not an exact science and you will frequently see availability in the fare buckets change.

The request to open up a seat in a particular fare bucket is usually done by a waitlist or queueing a request. On popular routes where BA is confident that it will sell out the F cabin, then a request to release further A seats is unlikely to be granted. It really makes no difference whether you are on an AONE* or some other A****** fare basis. The terms and conditions of the AONE* fares make it clear that carriers can capacity limit the available seats. On routes like LHR-JFK when you meet A0 you are unlikely to get much joy out of revenue management, unless their view of the likely F sales changes. You can always waitlist and standby at the airport. If an F seat is free and subject to anyone else with a better claim on the waitlist, they will give it to a waitlisted A pax.

"Regional availability" is the phenomenon that has been observed by FTers that A seats may be showing unavailable on a particular flight if the check is made in the US (for example by looking and Expert Flyer or trying a booking on aa.com), but showing available if the booking is made on ba.com or expedia. I don't know how many different regional zones there are, but I know from experience that sometimes when BA.com is showing no I class availability (for example) an I class ticket can be bought from aa.com. The same principle applies to changes to bookings. If you ring BA in London and ask for an A seat on an particular flight, you might be told that there is no availability. You may find if you ring BA in the US, or Spain or Switzerland that they can see availability in A class. Don't know why this is so, but it is a well-observed phenomenon on the BA board.