The key is: "Is it possible this is a WWII theater cut-down M1"????
Yes.
Anything is possible. However without any real evidence (providence) PROVING this, the rifle is nothing more than a cut-down M1 "Tanker".
Collectors and smart gun buyers "buy the gun, not the story". Without hard documentation (like a letter from the original GI who brought it home listing the serial number) along with some other verifiable evidence, the rifle has no value beyond what it is, which is a cut-down M1.
The M1 "Tanker" has been around since at least the 1950's when a gun distributor (Golden State Arms??) had a batch of M1 rifles that weren't selling.
On a trip to the Springfield Arsenal he saw the original T26 M1 and had the idea that that might help sell his rifles.
He had the idea of suggesting that the carbine version was ordered by General MacArthur for use by tankers in the Pacific.
Truth is, the short M1 was requested by European Paratrooper commander General Gavin, who hated the M1 Carbine, and wanted shorter M1 rifles for use in jumps.
Springfield Arsenal made up about two or three T26 rifles before the war ended and the need expired.
Their final verdict on the carbine M1 rifle was that it wasn't practical due to excess muzzle blast and recoil. One rifle was destroyed in testing and at least one other is in the museum at Springfield.
A number of rifles were converted in the field, but AS FAR AS IS KNOWN, all of them were converted back to rifles.
So, it's at least "possible" this is one of those few rifles from the Pacific, but as above, without HARD documentation, it has no value beyond being a cut-down.
Just because the rifle's receiver is NOT a welded scrapped receiver is no indication of where or who altered it. A good many have been built from intact receivers and most were not welded.
So, what you have is a family heirloom, and not much else without provenance.