Yes, during WW2 nearly all guns started to be made from stamped parts instead of milled. That happened especially in Germany first, because of a lack of rescources and the need to speed up production. Soon after also in Russia. But also the Brits went that way with some guns. Have you ever looked at a Sten gun? Its absolutly amazing how primitive that thing is built - and yet it works pretty well. Its made of 15 parts or so, and only the barrel and the bolt are not made of sheet metal.
About the conversion to semi auto: from what I understood, that is rather difficult to do. Because it is pretty simple to change the functionality of the trigger group to semi only - but then its just as easy to change it back again. And that they want to prevent. So they demand a solution that cannot be altered again, and thats not easy to do.
If you want the Thompson just as a display model, may be there are non functional replicas. Sure they will be more affordable, too.
For home defense its impractical anyway. You dont want to turn your house into a war zone, do you. For home defense you need something, that delivers the decisive blow with total reliability, and without the need to practice every week with it. Like a cut off double barrel shotgun or a good quality revolver, like a 4 inch S&W .357 or so with the right kind of ammo. All that is much more affordable than some fancy converted SMG or AR. You have those great gun shows in the US, don't you, where you can get 2nd hand stuff. Or from a pawn shop perhaps. And a old shotgun is not seen as such a dodgy weapon like a assault rifle, I assume. And yet it will stop everybody with one shot, no questions left. At a distance below 60 feet it never fails. Also not mechanically - stay away from pump actions. Nothing can jam with a double barrel, so you have always 2 safe shots at least.