"The main reason I want the center channel is that I have to turn the volume up on movies to hear the dialogue. Then I have to turn it down when there are special effects."
A couple of cautions here. Based on my experience, a center speaker may help a bit, but I wouldn't expect it to help a whole lot with regard to the issues you mentioned. Thus, I'd recommend you find a dealer who will allow you to try one out in your system and see how much it helps in dialog intelligibility. I have played around quite a bit with two different receivers with and without an actual center speaker.
First, if you have gone into the speaker setup in the receiver Menu and "told" the receiver that you have no center channel speaker, the receiver is already sending the center channel signal to your front L/R speakers. However, if you do get a center speaker, you will be able to raise the level of that speaker, but you can only raise the level of the center channel so much without screwing things up and I never found that to help all that much.
Second, with respect to having to turn the volume up to hear dialog and then down when bombastic effects occur, that is just the way these movies are mixed today. Most receivers include some sort of dynamic EQ or "night mode" feature to mitigate that a bit, basically reducing the dynamic range of the sound track.
Third, yeah finding a good center speaker is a problem when you front mains are fairly old. I guess you could find a stand alone B&W dealer and they may be able to help with matching. As to the YPAO, yes, it will still work with whatever center speaker you get, as it basically is setting volume levels.