Re: Are they all possible?
Agreed, but with two comments:
You can use a herd of people to speak metaphorically. It's not normal usage, but if you want to describe a scene and make it seem as though the people were moving (or being moved about) as though they were cattle, you could use herd.
Flock is applied to people in a church setting sometimes, as though the minister were the shepherd and the congregation his flock.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.