There is no "6 month window". The alternate valuation date, which is the 6-month period after the date of death, is used strictly when an estate tax return (Form 706) is required to be filed and the alternate valuation will result in a lower estate tax liability.
In this instance, the IRS would not be required to accept anything less than an appraisal by a certified real estate appraiser as evidence of the fair market value on the date of death.
