You're thinkin like I'm thinkin, but I think my version is better because I think yours is more complicated than it needs to be.
Mine's simpler. All I gotta do is pull the emergency standpipe out and 9 gallons will flow into the sump. Tee off the return and the return pump pumps it out to a bucket.
I don't think you'll be able to empty the skimmer section would you? Or maybe I'm misunderstanding something. That's a good thought though - to be able to pull the volume from the skimmer section. Actually, I could put a hole of some sort in the partition that separates the return chamber from the skimmer chamber, with some sort of blocking mechanism too, and then my return pump can pull the volume from there too.
Hmm...