How to establish a Two-to-One table relationship (PK-FK-CK)?
I'd say it depends on whether there's a connection between People and Project?

From what you've mentioned, I'd properly go with a:
Projects -&gt; ProjectsToMessage &lt;- Message
and 
People -&gt; PeopleToMesseage &lt;- Message 
type structure

(your bullet point 2)

Unless there's a releation between Projects and People in which case you might end up with some circular dependency which the database wouldn't like.
However in that case, I would then question whether a Message can be associated to a Project without it going through People.