You said you have it reported under Other reportable income, but you did not say that you entered it there as a negative number. I just wanted to be sure that the entry you made there is a negative number. The only place the loss gets entered as a positive number is under Other income not already reported on a Form W-2 or From 1099. These two entries for the loss on the excess contribution to the traditional 401(k) cancel each other out in your AGI, but make sure that the entire amount of the excess contribution to the traditional IRA is on Form 1040 line 1.
If Schedule 1 line 21 is showing a negative number, it appears that you've done all these things. I think is didn't fully understate that from your description of the entries.
