(decedent died in early 2003 and executor hired plaintiff to handle estate; plaintiff, attorney, suffered from numerous physical and mental ailments and missed filing deadline for Form 706 by more than three years; return ultimately filed approximately four years after death with payment of $138,179.27; estate’s income tax obligations fulfilled in 2006; withholdings sufficient to satisfy estate tax and income tax liability, but estate challenged imposition of interest and penalties; IRS Appeals Office abated Sec. 6651(a)(2) penalty, but nothing else; abatement of Sec. 6651(a)(2) late payment penalty does not estop IRS from assessing late-filing penalty; IRS not required to credit any income tax withholdings to estate tax).