1. Brass cases add zero strength to a shotgun barrel.
2. Maximum chamber pressure is called that because maximum pressure occurs in the chamber.
3. Steel inserts are called 'chamber adapters' or 'tubes' and allow a smaller gauge shell to be fired. A steel shell is a very bad idea because all shells must expand to conform to the chamber wall to properly seal the breech. Brass rebounds, as do metallic headed shells after firing so you can extract them.
All brass cases are not equal and the classic thin wall brass may provide different results than the thick wall RMC. I am not sure your statement in para 1 is correct. I note that Tom Armburst in his testing of the thick walled RMC brass hulls requires they be drilled as chamber pressure readings is radically off when simply measured through the solid brass hull. I am told this is because the more robust RMC hull does contain some pressure. I do not know the exact difference.
I also note there are variety of chamber adapters and not all are steel. I have often used chamber adapters from 20 to 410, 16 to 410, and 16 to 28 without having extraction issues.
