Yes, along with your grandfathers' sisters, if any.
Yes, your grandmother's sister is your great aunt.
He is your second cousin once removed. His children are your third cousins.
If your great grandmother was his great aunt you and he are second cousins, once removed.
Unless Kate Middleton's husband's ancestory includes some cousin marriages, she has eight (8) great great grandmothers-in-law. Some of them are, no doubt well known to history, but others less so.