The following section confused me:

Now - and this is a very important and fundamental idea in quantum mechanics - the amplitudes in cases 1 and 4 are flowing to the same configuration. Whether the B photon and C photon both go straight, or both are deflected, the resulting configuration is one photon going toward E and another photon going toward F.

It looks like you're saying that the result of the experiment is one photon going each way. It took about 3-5 reads to get from that to "the outcome of cases 1 and 4 are identical: one photon going to each detector." I'm not sure if that's just a reading comprehension failure on my part or if there's a way to rewrite the sentence to make it clearer (I might just strike the word "resulting").