It makes sense to close one rotunda - US Airways is down to 70 flights a day at LAS, and the most gates they use at any time is 14 gates around 8 p.m. If the airport administration really wanted to, they could close down both rotundas on Concourse A and just leave the walkway that leads to those rotundas (there are two gates on the left there right away) and still be able to maintain US' operations and not have it affect WN's operations in the B gates.

As for Concourse D, the plan as I understand it is as follows:

With the SE pier closing, there are three airlines affected: American, Allegiant and Northwest.

Northwest will move to the SE pier and use Delta's gates - DL primarily uses 5 gates at LAS, and they require 2 widebody gates (D38/D40). They have 22 daily flights, while NW has 12 daily flights.

Some peak times for Delta (not NW) at LAS - early morning: DL has 6 overnights (3 737-800, 1 757-200, 2 767-300). At late-morning, DL needs 4 gates (6 departures between 1040a and 1216p, the 2 SLC departures can use the same gate). The rest of the time, DL can get by with 3 gates, until the overnights start coming in at 6:19p). For comparison, NW needs 3 gates for their operation. They only need 1 for overnight, while they need 3 at midday (same times as Delta). Thus, Delta realistically needs 7 gates at LAS to run the full operation, and can probably get do it with 6 gates.

Thus, solution for Delta: give Delta D34 (which is Virgin's gate) and give Virgin D32 which is generally used by Allegiant. If Delta wants 7 gates, give Delta D32 as well, and move Virgin America to D31 (the former UAX gate).

American: Move Continental over to the SE pier, where their PC is. CO needs 4 gates, so they can have D33, D35, D37, D39 (the former UA mainline gates). That fills up that pier, unless DL needs only 6 gates - that leaves a gate for Allegiant. American takes CO's gates - they can run most of their operation on 2 gates, but really needs 3. CO's gates make sense. Thus, the NW pier looks like this: AA 3 gates, AS 2 gates (though they really only need 1...), F9 1 gate, Midwest 1 gate, Spirit 1 gate, JetBlue 2 gates. That leaves one gate in that pier.

However, with LAS being a CUTE airport, they can easily have Allegiant use gates in that pier, as YX only has 2 flights and Spirit has one flight that don't overlap, so that frees up a gate. So that's 2 gates there that Allegiant can use. Add in free gates on the new NWern pier and you might get 3 gates there. That might be enough to accomodate Allegiant.