You will need a good gunsmith. The information you seek is not readily available in published material.
You have answered your own question.
No, the Remington 870 slug barrel cannot be used with a Remington Sportsman pump shotgun. Each model of shotgun is designed with specific dimensions and configurations, including barrel interchangeability, so they are not typically interchangeable. It is important to use only compatible barrels with your specific shotgun model to ensure proper fit and functionality.
NO! The 1187 barrel was for remington shotguns,the auto-5 shotgun is a browning shotgun.
no
no
AFAIK, no.
no but it will fit a mossburg 500
No there are ways, it invalves drilling holes to vent gas witch operates the gun.
IF it is possible, stick with Remington parts.
700 ADL Long Action
Without seeing it, almost certainly not.