It's a first-come first-served basis. The first person that triggers a download after 00:00 decides on the first image that's being made available.
Correct. However, since no-one usually manages to hit the button at the exact hour, there are usually a few seconds difference (for example, the second image might become available at 01:00:15, the third ast 02:00:30 etc.) These small intervals accumulate over time. By now, it's already accumulated to 25 minutes after the full hour.
You can know if you triggered a new download when it takes unusually long before any result shows us (usually 10 to 20 seconds).
Correct. Usually there are 24 different editions available at any time, as long as there are unused product keys available on the database, from which new downloads can be generated. Usually, Win 7 Pro and Ultimate are used up first, then the Home and OEM editions, and Office downloads last the longest. After 6 days, all keys are used up, and after 7 days, the last downloads have expired.
That's because I only have five product keys for Win 7 Pro on file. Each one can generate one download link per month. Once they're used up, that's it. Next month, next chance.
However, Win 7 Home Premium COEM is currently available in Dutch. The link expires at 9:10 tomorrow morning. You can easily convert that image to Windows 7 Pro.