My problem seems to have been resolved on its own. It definitely seems as though it was on CRA's end, as I attempted to submit the exact same, unaltered tax return the following morning and it was accepted no problem. I would suggest simply waiting a day and trying again as I did. I'm not sure if filing it in the morning had anything to do with it, but I did so on my final attempt to file and it worked. It's possible that the CRA is just receiving too many NETFILE submissions at once and that filing during non-peak times would be best.
Cheers!
