Thanks. I wondered about the 2011 but everything from IRS says three years, although it does sometimes specify that's for a refund. (I suppose most people wouldn't voluntarily amend a return five years later if there was a tax due.)

Filing by paper is OK this year. My main concern is getting the account straightened out so the same thing doesn't happen next year.

What about the repayments they made from 2011-2017 to the basis? The 2011 Form 5405 asks for repayments made in 2010, which of course is $500, but they've paid $3,000 in additional repayments since then. They should get credit for that somewhere.