Any other examples of this phenomenon in Quran using proper nouns? Give me one example of a placename in Quran that allows swapping of the M and B. Does Quran call Ibrahim "Imrahib"? It doesn't. Does Quran call Musa "Busa" as well? It doesn't. Where is your proof this change in root form still allows to mean the same identification?
Peace
At the time the Quran was revealed there were many tribes of arabs this message was being conveyed to, and other than them were many non arabs who either lived there or traded there, who would have accepted islam. You are assuming that none of them used bakka for mecca. It is quite possible that many did do so, and hence the use of both words. This is more plausible than your version which puts bakka to be in Jerusalem, which no historian or actual historical record ever shows to be true.
There was never ever a bakka in Jerusalem but there was a Mecca in the Hijaz. What is more plausible?