An extension can be granted on the Federal Inheritance tax-don't know much about the Tn. state Inheritance Tax but I'm sure there are extensions granted for them also if its necessary.
As far as Minnie Mae I don't think anyone tried to defraud her. (IMO) she was taken care in every way until she died-their was no other provision for her. But their was a provision for taking care of Lisa, or any lawful children after Vernon and Minnie died-which included closing the trust on her 25th b-day and turning over the estate to her. If Minnie had lived to 150 she would have been taken care of and provided for. But I see no provision for her to actually inherit property, or for the estate to be split between her, Vernon and Lisa at some point in time.
I know some people may think "she was left a third of the estate which should have been handed down to her descendants"-there is nothing in the will that says this (that I can gather from reading from the provisions.)
But she and Vernon would would have been take care of -forever in the style they were accustomed too if she could have lived forever. After they died the "estate in trust" was set up to be divided among any lawful children equally when they reached the proper age.
If someone related to Minnie can find a provision that states- Minnie and her heirs were to get a third of the estate-they need to take it to an estate lawyer and get his opinion. If that lawyer sees any hope of that being the situation he will take it to court in a heartbeat, on a contingency basis. His fee will cost them nothing- unless he wins the case then the agreed on fee (and expenses) will come from the settlement with EPE.