1. You’re changing goal posts. A semi-auto shotgun allows quicker follow up shots than the pump actions that were being described, and are a better option than pumps for home defense... if you don’t mind a higher risk of collateral damage.
2. I have quite a bit of practical experience in combat, urban warfare and clearing houses. My comments do not come from academia or video screens.
3. First of all, the German complaints you mention were from WWI, not WWII. Even in WWII, they didn’t have “some” bolt actions. The majority of German soldiers used bolt action rifles. In WWI, with the absence of SMGs, there were even more. And the MGs were used for fire and maneuver outside of trenches, not inside the trenches. Lastly, no it wasn’t just one or two Americans with shotguns killing or capturing companies of Germans or rendering them useless. Shotguns were an effective tool when complimenting squad and platoon maneuvers for clearing trenches in WWI. That is true. But you’re overemphasizing their role in WWI, and they weren’t used nearly as much in WWII.
4. For the Marine Corps TTPs, I don’t know how they currently use the shotgun. I wasn’t in the Corps. However, in the Army and in most of the SOCOM elements I worked with, the shotgun has been reverted to a breaching tool for the later ~3/4 of the GWOT. I will say that I like a lot about Marine Corps ideology, but they’re sometimes stuck in their mindset with weapons. They maintained the 20” M16 as their primary Infantry weapon for far too long, as an example.