1. You can, particularly if you already have your AA number on the reservation. If you don't, you can add your AS number in Manage your reservation on the AA website or during booking.

2. Yes, you can also do that if the flight isn't credited elsewhere. Save your boarding pass and ticket receipt, which is a good idea in any case until the flight credits.

Your assumptions are correct although it's possible that if you have your AA number on there to get MCE at no charge, an AA agent or the system may boot you back to a main cabin seat if you change your frequent flyer number to AS. Also, while you earn a 100% RDM bonus as a MVPG, many AA economy fares earn less than 100% and that bonus is on those reduced miles not actual flown miles. For example a round trip LAX-Tokyo in Q (~10K actual miles, Q is 25%) will earn ~2500 EQM & ~5000 RDM.