a certain amount of expertise in the vagaries of the market. So I pose this question: Are there circumstances in which insiders are precluded from buying shares on the open market? Can they buy if they have knowledge of significant events not available to the investing public (that would be me)?

In too many cases I have seen the old "why aren't insiders buying if this company is so good" used as a red herring. Not saying you are doing so, but I also don't believe the fact that insiders are not buying is necessarily significant.  JMO