I believe I'm descended from the Ross family you describe (and therefore also Christina Grant). However, some of the details you mention don't quite match what I have - for instance, I have William Ross married to Christina Grant around 1847 (both born around 1827/1828), with both of them born in Nova Scotia (not Scotland). It is probably William's father (also named William) who came from Glen Urquhart to Nova Scotia in 1818.
Also you might be interested in a book, available electronically by searching on Google, called Pictonians At Home And Abroad. I didn't see Peter Grant in the index, but there were several Grants.
Contact me at treeBecky@yahoo.com
if you'd like to continue the conversation.