Yaakov Avinu Lo Mes is completely irrelevant to the discussion here. There is extensive discussion among the poskim of many generations regarding the entry of Kohanim to Kever Rochel, and the poskim are near unanimous that they may not enter at all. I am not familiar with how they deal with Meoras Hamachpela and I don’t recall whether it is part of the discussion, but there is ample basis to recognize the marked kevarim there are not at all the real locations of the Avos, though the proximity is not questioned.
The reality is that the pronouncement of anything to permit a Kohen to approach a kever must be something extremely solid among poskim. The hergesh of chossid is unreliable for this, and those Kohanim who approached the aron were undoubtedly oveir their issur. This is an example of the deification I described that was seconded by Phil.