@yankel berel
If they are an integral part of klal Yisrael, and our brothers, then that would mean that there is in fact a very large theological common ground. If there really is no theological common ground at all, then not only could they not be considered an integral part of klal yisrael, they would not be able to be considered part of klal yisrael at all, and by extension, not our brothers.
Since I assume that you don’t believe the second option, that would mean that there is in fact common ground upon which to argue, rendering your entire post moot by its own premise.