Someone did not file a schedule C for the 2014 and 2015 tax years although she incurred expenses.

As a result the IRS levied taxes based on the 1099s reported to her.

Questions:

1.  Is it too late to amend these years' returns and add in the expenses?

2.  If, for some reason, she entered into an installment agreement with the IRS (she can't remember for sure), will she be able to file the amended returns?

I've looked around.  It seems like they are saying 3 years is about the limit.  Asking if anyone has any tips and if you've tried amending a really old return, similar to this?

Thank you!