Your question has a load of information in it, so I'll try to offer what expertise I can based on the information given. But I am confused by your legal description: "L2"? Are you trying to say "lower half"? That would be S2 E2 NW, and that would contain only 40 acres, not 157.64. And either a creek/riverbed borders one of the boundaries of the land, or this Section 7 is in what is called an "irregular section" shorted acreage during the original survey to adjust for the curvature of the earth. If it wasn't for the "L2" I would look in the OCC website for you to see what producing wells I can find on or near that legal description.
That said, I assume the "probate document" you're talking about is a probated Will?
You also ask if there is a reason for a new lease. There could be several reasons. One possible reason is if all of the minerals were leased at the time the first pooled unit was formed, but the Declaration of Pooling filed in Payne County either described incorrectly, or inadvertently left out the lease covering your cousin's interest (a different lease from yours because it likely was willed to him from an uncle or an aunt? And not from the same ancestor from which you got your interest?). Another possible reason is that the lease under which your cousin owns mineral rights contained a horizontal Pugh clause (a "depth Pugh" causing depths below the deepest drilled or producing depth to automatically expire on the trigger date specified in the lease). Your lease probably does not, if it was a 1983 lease. Many leases back then contained a vertical Pugh ("surface Pugh" clause) requiring surface acreage outside a producing unit to expire, but didn't contain a depth Pugh also. So if a new well was proposed for a deeper depth in which the lease covering your cousin's interest had expired, a new lease would be required, but the legal description in it will clearly state "LIMITED TO" and then state the depths, or say "LESS AND EXCEPT" and state the depth from the surface down to the base of the currently producing horizon.
You say that lease holder "A" says that they only have 40 acres out of the original (157.64 acre?) lease because the other 120+/- acres were sold to others. If your 1983 lease didn't have any kind of Pugh clause in it, putting 40 acres of it into a unit will hold the other 120 acres, and someone else could be holding them for future drilling.
It does sound like you have money that has been "escheated" as unclaimed property (paid over to a state government agency as abandoned by your grandmother). What you may not know is that if the "holder" never had any address for your grandmother (not even the state of her last known residence), the money would have been paid over to the state in which the holder is incorporated (or franchised). So without knowing which oil companies over the years might have paid money to the states as unclaimed property, you will need to literally sit and research for lost money in your grandmother's name IN ALL 50 STATES. No kidding.
After this, your information gets very murky. Who signed a lease in 2005? You? Your cousin? If it's your cousin's lease, it very likely could have a 3/16 royalty rate--the modern "base rate" for leases. Your 1983 lease, however, would continue to be paid the 1/8 royalty reserved in it.
A history of "the well" should be available in the OCC website--and like I said, if I knew what you meant by "L2" I could look it up for you, see what information I can find. Payne is in north central Oklahoma, where successful horizontal oil wells are coming in. You need to find out the status of your mineral rights, find out who the operator(s) are of the well(s) on or near your land (and your leased land is pooled in).