Lisa Foster herself admitted that the mailing of the life insurance policy premium and the oil lease papers was done from the WH the day before the crime. (Carl Boyer's story about Lisa in NYer Magazine, Sept. 11, 1995). That business about mailing the letters was first mentioned in the FBI interview report of Deborah Gorham, VWF's personal secretary at the WH, who actually mailed the letters. It wound up in Fiske's Report on the death, though the words were modified to reduce the index of suspicion. It was also reported in Chris Ruddy's basic book about the case, "The Strange Death of Vincent Foster."
You are probably correct to say that, "I don't think those benefits have been lost if someone other than Lisa had killed him." I'm not an attorney, but I'd guess that if it were shown Lisa were a part of a conspiracy with many co-conspirators (which is, I believe, the truth), the benefits would be (legally) lost as well.
As I indicated, (1) the mailing of those papers on the day prior to the event is highly suspicious of L's foreknowledge; (2) L's activities on the date of the event are very sketchy in her FBI interview (Although she didn't admit to it, it's quite likely that she supplied the "family car" to whomever drove it to the park to fake the "suicide" - no report of of a stolen vehicle and no evidence of any family member in the car at the park.); (3) she would have had a large finincial stake in a "suicide" determination; and (4) her cover-up activities were numerous and persistent.
I can’t see where Lisa would have any involvement in the murder of her husband, certainly not for the insurance money. The fact that they were in marital turmoil because of the move from Little Rock to DC, I can see perfectly. It would be a difficult transition. If she did anything I think it would have been to refrain from yelling “THE CLINTONS MURDERED MY HUSBAND” in a very loud voice. She would have agreed to be quiet to protect her children. Then the son of the man she married died in suspicious circumstances, as I recall.