I'm not smart enough to follow the fluid dynamics line of thought, but does it not go:
- Path to 5 fills but is blocked.
- Path to 4 fills but is blocked.
- Path to 6 fills but is blocked.
- Path to 3 fills but is blocked.
- Water escapes and starts to fill
2.
- Water backs up to path to 7 but is blocked.
- Water fills to paths to 1 then 2 but is blocked.
- Water may back up to path that allows escape over to 1, but by then the path to 2 is still filling 2.
- Maybe there's a backup and water spills over the top to 7 and 1, but by then 2 has been filling the entire time
My vote is 2.