I actually wondered if maybe Uriel forcing Dean to give Anna up for Sam was for real, and they only came up with the rest of the plan *after* that. Because the plan wouldn't have worked if they couldn't be sure of *both* sides being there, and would they really have been sure that they would have a way of getting the angels there at the right time? (Ruby *called* Alastair and tricked him into coming, but Uriel came to Dean by himself, which isn't something that they could have any control over.)

But maybe that's just me trying to rationalize it all away. ;)