Opus_17 helped answer many of these questions for me, so I thought I would share where I can.
The reason box 1 is empty is because you repaid the bonus in the following tax year. The corrected W-2C 2018 is only to represent what was changed in that tax year. If you repaid the bonus in tax year 2019, you will need to file the 1341 for 2019 tax year to claim the credit of overpaid taxes in 2018. Honestly, the whole W-2C just causes confusion and why you (and I), think we should amend the 2018 tax return to claim the refund.
Anyway, that's why box 1 is empty, it's supposed to be, as you did receive that income in 2018. I believe the change in the other boxes, is because you may have filled out a FICA form and your employer would have calculated an amount to refund you. In my case, I had to repay $10k, but actually amount I repaid was around $9200, because I signed the FICA paperwork which allowed them to get the refund back on the medicare/Social security taxes, which I believe is what they passed back onto me (why I paid $9200 instead of $10k). So I guess the next step is I need to file the 1341 with my tax return for the year in which I paid back the bonus.
It really is such a pain. Of course the IRS makes things difficult to get the money back. They could have easily created a field in the form to where you simply indicate a repayment amount and your employer could provide some tax document to prove it. I assume they just figure most won't bother and they keep the change.
Good luck!
