Okay, roac, I'll bite
The main support for the theory of the Nazgul-attack is the description of the Nazgul attack on Crickhollow that immediately precedes the discovery of the slashed bolsters. Since that is the only place where we see things from the Nazgul's point of view, it is rather significant, don't you think? It seems to set up the next scene - once the Nazgul discover that the Ring has left the Shire, what will they do? Will they attack Frodo in Bree?
I think Tolkien leaves this question ambiguous because the hobbits really have no way of knowing what actually happened. If Frodo does not know the answer, he can't write it in his book. It is an unsolved mystery of LotR, if you will . Surely the Bree-folk debated it for ages, and came up with various solutions. While the attack by Ferny is what Aragorn expected, it is by no means certain that that is what actually happened.
As an aside, I was surprised by your understanding of the hobbits' rooms. I have always pictured a 'suite' setup, where the bed rooms were connected to a common room, or even all four beds in one room (though clearly that is not correct). I had not pictured 4 separate rooms with four separate windows and doorways into the corridor. One reason why not is because it seems that the damage caused here can be surveyed rather quickly. It is as if they open one door, and see a mess. I don't picture them going to each room in turn and seeing a new mess each time. Of course, the text does not really specify, and I have never been to a real inn .