There are only two possibilities - either Kraus wrote this march first or Mozart did.
You assume Mozart did. So do most other people. But it could be the other way round. There is no real fact to contradict such a possibility.
I will check the manuscript to see if the march exists in any form other than the full score. If it does not then it must realistically have been written only weeks before the premiere. For Mozart expressly says at that time that he has not yet written the score.
This would certainly explain why Kraus does not mention Mozart at all in 1789 - it was never Mozart's march in the first place.
There is too the possibility that the march was only added to the Vienna arrangement of 1786. That too must be considered.
So, on balance, I don't think this is quite as ridiculous as you seem to be suggesting.
It's not until these basic assumptions are questioned that the chance increases of there being a solution.
Rgds
Robert
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