Sorry to hear that Rob. There can be no excuse for making guests feel embarrassed just because they didn't quite follow etiquette but even so. As you say, he should have just laughed it off and got on with it rather than act in that way. I can appreciate that pressing the flesh and having to force a smile can be grating after a while but that's his job and part and parcel of being in command of a passenger ship.
Your experience was in complete contrast to Bernard Warner on QM2 (yes, the one I didn't get to shake hands with lol). The cruise dvd shows him greeting a passenger who didn't know whether to shake his hand or salute him and it ended up resembling a Monty Python sketch (you have to see it to believe it but needless to say, it still makes me laugh whenever I watch it). Pure comedy and fair play to him for injecting some humour into an otherwise formal occasion. I met him later that evening on the stairs while heading into Dinner and we exchanged pleasantries (or whatever you exchange in the prescence of a Commodore). A truly nice guy, as I'm sure most are.
DP on the other hand, has been accused of being unapproachable and rather stand-offish with passengers. I can only speak as I find as my experience of him the both times I sailed with him were fine yet others didn't feel the same way. My last cruise aboard QE2 coincided with his and I noticed that some of the guests at the CWC Cocktail Party were giving him gifts and cards afterwards, so they certainly seemed to hold him in high esteem. Christopher Rynd was another I recall who some couldn't warm to yet others had no problem with.
As in all things, our feelings are largely influenced by our experience, which in your case is clearly understandable, whereas in truth, mine differs slightly. I'm not saying his actions towards you are excusable for they are not but while he may not have the same friendly disposition as Bernard Warner, I found him ok. Maybe you were just unlucky, either that or I was!