If Mark had written such a work in Aramaic, it would not have survived. Your argument is that since the book exists - the later gentile Greek church having preserved it, as it did e.g. Josephus, Philo of Alexandria, the books of Macabees etc. - it can only allow the interpretation that this later gentile Greek church put on it. Any other interpretation would be possible only if the work were in Aramaic and thus no longer extant. (Consider for example that it is perfectly possible that there were Aramaic letters Paul sent to the group(s) he presumably founded among the Nabateans to whom, as gentiles, his vision apparently first directed him -- but of course we would not have them. Your argument is as if one were arguing for a Paul - as - within - Judaism view by saying, he would have written in Aramaic; but it seems perfectly possible that he did.)
If Mark had written such a work in Aramaic, it would not have survived. Your argument is that since the book exists - the later gentile Greek church having preserved it, as it did e.g. Josephus, Philo of Alexandria, the books of Macabees etc. - it can only allow the interpretation that this later gentile Greek church put on it. Any other interpretation would be possible only if the work were in Aramaic and thus no longer extant. (Consider for example that it is perfectly possible that there were Aramaic letters Paul sent to the group(s) he presumably founded among the Nabateans to whom, as gentiles, his vision apparently first directed him -- but of course we would not have them. Your argument is as if one were arguing for a Paul - as - within - Judaism view by saying, he would have written in Aramaic; but it seems perfectly possible that he did.)