At this point you don't know if you will have any deduction at all on your 2018 tax return. There are multiple possible scenarios where you will not have the deduction. Therefore it would be a mistake to reduce your quarterly estimated tax payments on the assumption that you will have the deduction. You could end up with a big tax bill plus penalties when you file your 2018 tax return. Your original plan to wait "for everything to even out" is the best approach.
The letter you got from the company might be useful in the future if you claim the deduction and the IRS later questions it. But the letter is useless for deciding whether you can adjust your quarterly tax payments, because it doesn't tell you how they are going to report the repayment.
In the thread from two years ago that you referenced, the OP never actually said whether he knew at the time he received the bonus that he might have to pay it back under certain circumstances, but it's extremely unlikely that he did not know. If he did know, then the information that Hal_Al posted about the credit is mistaken. He could not claim the IRC 1341 credit for the same reason that you can't, as I explained above. He did not report the income "under a claim of right."
