So again, there are two possible scenarios here: The alpha backers play new versions in a parallel universe that gets scrapped when the update is released, or they play them on the "real" universe while everyone else is shut out. Which one it is, as far as I can see, has not been established.
Indeed, but does it matter? My guess is that it will be the former as the latter doesn't make any sense. The progress will be scrapped multiple times as time goes by, this would just be another of those times.
The other option is that they run multiple servers (as they do now currently) and only one of them is updated to the latest alpha build. Alpha players then use this server, whilst beta folks use the other server(s). When the build is released to the beta folks, the other servers are then updated to the current state of the alpha build.
I don't see any point in them doing that though, as it would mean (for some) that alpha have had two weeks "progress" - and even though that's irrelevant at this stage, it doesn't seem to stop people moaning about it.