It's possible to have second/third/fourth generation siblings born on both sides of the 2009 cutoff, but only those born after 2009 are ineligible for citizenship by descent. This might seem cruel and unfair, but all of these generations were born and living outside of Canada. The only way to guarantee citizenship is if their parents gave birth in Canada; so if their citizen-by-descent parents wanted Canadian citizenship for their children after 2009, then they should have planned to give birth in Canada. Some countries have stricter by-descent laws, some are more generous; I think Canada's law is in the middle, maybe leaning slightly generous.My thought/hope was that the limit only applied to people born after 2009, to prevent people (and their children) continuing to live outside Canada from claiming citizenship; in effect saying, hey we'll give you a pass for the 20th century, but if you want to continue calling your multiple generations Canadian, you have to come back to Canada. And I guess they still did give you a pass while living multiple generations outside Canada if you had made the simple filings back then, which isn't a ton to have asked.
Under the British law, a woman did not lose their status automatically unless she was naturalized automatically through marriage. It would change the pathway, but it would still not have allowed you or your mother to be able to claim citizenship from him today.I don't have actual proof that my GGM naturalized at the same time as my GGF, or at all. I also have records of entry into the US for my GGF, but not my GGM. I know it's a pretty unusual case, but could that change the scenario at all, or would one parent naturalizing cause loss of British Subject status?
The earliest your grandfather could have claimed citizenship through your great-grandmother would be with special grant under 5(2)(b) of the 1977 Act; it basically says that if the person was ineligible for citizenship by descent under the 1947 Act because of the parental restrictions, then he/she can now apply for a grant effective to date of application (not date of birth like by descent). Even if he did this, it would be after your mother was born, so she would not have gotten citizenship under it. Those that applied for and gotten the grant were converted to 3(1)(h) citizens under the 2009 Act; unfortunately, parents under (h) are among the list of first-generationers that can not pass on citizenship to their children. So even if your grandfather gotten citizenship that way and retroactively dated to his date of birth under (h), you and your mother would not be able to benefit from it.