I believe that there is some well documented information about your Alexander family on the Chester County GenWeb page. The information was posted by King Alexander who seems to be very knowledgeable. You might want to check out that webpage and/or try to get in touch with him.
I am wondering also who the John Alexander was whom David sold his New Munster land to. If it turns out that John was another son of David, that would be very interesting. I would like to look up copies of the complete deeds to see just how they are worded.