Well those aren't really too relevant to the theory but I think that both of them can only be answered by speculation since no real evidence is present. At least not that I know of.

And assuming that the Happy Mask Salesman is some kind of supernatural being who can live for thousands of years (Thus being the skull kid's counterpart) and that he predicted / knew of the moon falling would pretty much explain both things. He's appear in front of Link moments before the end because he probably knew of Link's past and "monitored him". Even if he didn't knew about Link he did met him in a strange form and asked him to help him get the mask so he probably kept a close eye on Link all the time.

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