Another expert here - below is the additional information/clarification you requested.
Just to reiterate, Spouses who are the sole designated beneficiary can:
1. treat an IRA as their own, or
2. base RMDs on their own current age,
3. base RMDs on the decedent's age at death, reducing the
distribution period by one each year, or
4. withdraw the entire account balance by the end of the 5th year
following the account owner's death, if the account owner died before the
required beginning date.
If the account owner died before the required beginning date, the surviving spouse can wait until the owner would have turned 70½ to begin receiving RMDs.
As to treat the plan as an inherited/beneficiary plan and take required minimum distributions based on her life expectancy and beginning the year that follows the year Jack would have turned 70.5. If the owner died before his RBD, base
required minimum distributions for years after the year of the owner's death
generally on the spouse's single life expectancy as shown on Table I in Appendix C.
You are the owner's surviving spouse and the sole designated beneficiary. The owner would have turned age 70½ in 2013. Distributions begin in 2013. You become 69 years old in 2013. You use Table 1. Your distribution period for 2013 is 17.8. For
2014, when you are 70 years old, your distribution period is 17.0. For 2015,
when you are 71 years old, your distribution period is 16.3.
As to deplete all funds by the end of the fifth year following Jack's death, a beneficiary who is an individual may be required to take the entire account by the end of the fifth year following the year of the owner's death. If this rule applies, no distribution is required for any year before that fifth year.
My point of reference is Publication 590 which can be found at the following:
I hope this additional information is helpful to you. Thank you.