1) longest prefix match and AD is only for installing prefixes of equal lengths that were learned by 2 different sources.
2) you can do on both or on one only and no the summary is not propagated between ABRs but of course the other ABR knows about the summary as it is part of the area where the summary is injected.
1. If you had the following two routes in the table:....
If a packet then hit the router with a destination of 10.1.1.2 then entry B. above (10.1.1.0/30) would be used as it has a more specific prefix.
2. If you were summarising from one router, you would want to summarise from the other aswell other wise one router will advertise the summary route and the other will advertise the more specific routes which would be pointless. it would possibly stop utilisation of one router in favor of the other due to the more specific routes aswell.
Hope this helps