It’s good that you asked the question. You were assuming something that isn’t necessarily accurate.
As noted, you need to contact Amherst about what they want for a special circumstances consideration (due to the reduced income). They will tell you what documentation they want…and when.
If this is an ED application, it’s very possible that this special circumstances consideration might not get resolved in time to make a decision about accepting an offer of enrollment. BUT you absolutely can ask for an extension on that decision deadline for ED students.
It might all work out just fine…if the net cost is such that you two parents can split it and pay the bills.