Another rule I've followed is that the -est and -eth forms of a verb should be either both included or both excluded.
In this case, cometh is already a valid Chi word, which would warrant comest being admitted even if there weren't all of those instances in Shakespeare and the Bible. So comest will be accepted in future.
Cometh seems to have more currency in current writings, partly because of the saying "Cometh the hour, cometh the man", whose origin appears to be uncertain. And then there's Eugene O'Neill's play The Iceman Cometh, which no doubt was the inspiration for the title of the Flanders and Swann song that anona linked to.
