I did an AA Platinum Challenge recently and was successful. Now I am wondering if I have to chase up a OW Sapphire Card? Also i am planning some BA flights for Nov. and wonder how I use the new status in my reservation. I am very interested in taking advantage of the better seat access for Sapphire status but am not so sure on the way to use it?

There's no such thing as a OW sapphire card. If you have your AA Platinum card then that is it. There is a blue oval on it which indicates your oneworld status. When you book flights with any oneworld airline you give them your AA number and they should automatically see your status from that. You will also require your AA Platinum card to access other oneworld airlines' lounges.

So if I fly BA and earn BA miles on an LHR-JFK then the benefit of my AA documented Sapphire status can not be be used. Is it fine to use the AA Platinum number on the BA MMB a (I have to check if there is actually that feature allowed on line) get the early seating benefit and then revert back to my BA number to max the miles earning?

It's not clear to me why you're wanting to credit the miles to a BAEC account which I assume you also hold...however...

There's no problem with flying on BA, crediting miles to BAEC but using your AA card to get into the BA lounge. Where it risks going wrong is if you get your AA f/f number into the booking for the purposes of (e.g.) seat selection, that the flights will also post to the AA f/f account. What you need to do is prior to getting on the plane, ensure your BP shows your BAEC number and not the AA one - otherwise you'll have to sort out the mess afterwards (which could be tricky). You should ensure that your BP shows the f/f details of the program you want the flights credited to; you should be able to get this changed at the airport (i.e. at check-in, or at the lounge).

The reason for posting these miles to a BAEC is simply because I don't want to lose with the 25% BA flight AA miles reward scenario. I lived in the UK for 5 years and have a nest egg of 150,000 miles so redemption just needs some thought given to it. My attention should be given to clearing things up on these two programs though.

You can't earn AA miles on BA transatlantic flights anyway further if you are flying in discount economy (I'm assuming this since you thought you were only eligible for 25% miles accrual on AA) you will earn 25% of the flown miles on BA.

From aa.com: