As was mentioned, all you can really do is try out filing Separately.  Although that disqualifies the credit, the lower-income spouse may have a lower repayment of their 50% portion of the Advance credit.  But filing Separately may cause other tax, such as taxing some of the Social Security.

In the event they have any "earned" income from a W-2 job or self employment profit, contributing to a Traditional IRA may lower MAGI.  If you can get MAGI below 400%, that is what you are aiming for.