Not going to discuss this with an expert since I am not one.

Still many of those myths do have common themes and stories but also sometimes big changes in which god exactly partakes in them and their gender from cultures that we do know have had contact, sometimes by trade or war/conquest and where if we go early Mesopotamian times or earlier documents tend to be extremely rare. It would rather be weird if there had not been cross influences as opposed to the American continent, where for any contact we would have to go back way further in history for there to be a land bridge. In which case cultural contamination would mean those myths go back way earlier as assumed and it is way more likely similarities come from having come to same answers to dealing with a certain lifestyle.
Isis is for sure not a copy of Inanna, hell one might even say there are major changes in Inanna herself already in history of Mesopotamia and pretty conflicting myths. Considering some of the myths though in Egyptian culture, it is unlikely that those myths surrounding Inanna did not influence theirs. Whether that was directly or by a shared even earlier origin of those myths or a third party with similar myths that influenced the Egyptians, who knows. It is not like we know that much yet of very early Egyptian culture or pre-Egypt either and are still learning partly due to there not being documents and even if some are there they tend to be declarations of very powerful rich people with their own political motives and better be careful to take them at face value.