1) If the Hold expires then in a general sense the tickets go back into inventory, but that may not be evident to you. Fares could go up or down. There is no risk-free element to this, if you let the tickets go it's a gamble as to what happens next.
2) I don't know about AARP, but for similar areas such as Veterans fares, taking advantage of that means you don't get the Hold facility.

I would be surprised if you needed the Google front end, as far as I know a direct BA.com search should throw up the same thing, but if there were only 2 in a particular fare inventory then you may have had the right result anyway. The Teen business is to do with APD, which depending on what you are trying to do maybe irrelevant.

The other thing to bear in mind is that you do have a 24 hours hold for free anyway, if booked direct on BA.com, giving a full refund. This also allows amendment (e.g. to ages and names) for free, changes to flights may result in a fare difference.