Elizabeth:

Thank you for your thoughtful reply.

I do understand that comparing relationship predictions between platforms is problematic, which is why I've asked the grandmother if she is on GedMatch. However, I have compared the grandson's kit with my own and he matches me on 11 segments, sharing 218 cM and shares a bit more DNA with my brother. However, even across platforms and methodologies, I would expect if our MRCA was through his grandmother's line, Grandma and us would be much more closely related than 5th - 8th cousin, even if we share more DNA with the grandson (as you explained in #2).

Because there is such a gap in the predicted relationships, I do think #3 is the most likely scenario. Hence, until the grandmother responds and is willing to share her genome, I can safely eliminate Grandma's line while searching for an immediate ancestor to my dad.

Thanks again!