It seems that we can't select a non-milestone project task as a parent task. Is this correct?

Correct

@Deepthi noted. Thanks. Do you happen to know if it's possible to run a workflow (with server triggers) against the project tasks created via CSV import? Per testing, it doesn't seem to be possible. Maybe because it just makes use of the simple import wizard instead of the Setup > Import/Export route. I hope I'm wrong and I'm just missing something.

Haven't tried that scenario yet.

Thanks anyway.