Seeking assistance for 1040-x (MFS to MFJ)

Hello folks, I have had major frustrations with tax contractors and have received different responses, so seeking this community's help.

Summary/Context:

• Married as of 12/31/2024; divorced July 2025; I am 100% 1099 income, and wife was W2 for 2024
• Joint tax debt from 2023 – per divorce agreement, I assumed 100% responsibility for this (which IRS does not care about, as it is joint debt)
• In 2024, my ex and I filed Married Filing Separately. She received a refund, about $7600. The refund was seized by IRS and applied to 2023 joint debt. I personally reconciled/repaid this to my ex to honor divorce agreement.
• In 2024, we still shared health insurance. Her income was low, mine was higher, and she claimed our two kids as dependents (also per our divorce agreement). Therefore, I got penalized for the the healthcare premium tax credit, underpaid estimated taxes a little, and had a somewhat significant balance.
• Created payment plan with IRS for 2023 debt; as of January 2026, this debt was completely paid off, and monthly payment plan is now going towards the 2024 balance.
• My ex has agreed to the idea of doing a 1040-x amendment for tax year 2024, to switch filing to Married Filing Jointly.

Questions:

1. When doing the math for correcting/combining the numbers on the amendment, what do I do with my ex's 2024 refund that was seized by the IRS? How and where do I put that in the calculations?
2. Can I add the three monthly payments towards the 2024 debt I have made thus far this year?