I wonder if the olfactory cortex is more sophisticated or simply larger. I have a large dog and I recon the surface area of his nose is probably the same as my house or larger.
I'm pretty sure most dogs understand "no" (and the equivalent word in other languages is often 1 syllable). I haven't done anything approaching a study, but I've trained many dogs in my life and my experience disagrees with you. Some other common words they know really well include: treat, sit, off, here - basically anything you teach them.
"Who's that" is something we always said when approaching someone we recognise, and now if she hears those words - even totally out of context, mid way through a conversation - she gets excited and goes to look out of the window.