No, it is not too difficult to amend the 1042-S (but it IS more work to amend than to have just filed a correct one in the first place).
Did your original 2016 tax return include the 1042-Ss? If so, I don't think you would need to include them at all. If you are adding them to the amended return, yes, you need to include them. But as I mentioned before, without your SSN on one of them, they may not credit you the withholding on that one.
