You would definitely owe interest from the original due date. You may be able to call Cincinnati and get someone on the phone who can help. I've called on other issues and they'll tell you as much as they can without having a POA on file. The phone number I have is 1.866.699-4083. You could also call Kyle Martin with IRS - he speaks to a lot of groups so he's a natural contact person. His number is 510.637.4549.
Good luck!
There is a new estate planning forum that you're supposed to be directed to. If you have this one in your "favorite places" it will be going away soon. Check the CalCPA website to be linked to the new forum. Mary Kay Foss