Yes. Slug barrels have a different "Choke" then shot barrels that make them more accurate when firing slugs, but you can fire slugs with a stock 1200 barrel.
I have one that I would sell.
No
At this time no,there are not any available.You could have a model 1300 slug barrel redone and have the gas ports added to this barrel.Then it will work fine in your model 1400 Winchester shotgun.
As long as the slug barrel is smooth bored(does not have rifling) then it can be used with bird shot.
You can try Hasting's. They make after market barrels.
100-300 USD depending on condition
Not anymore but I hear you can take a 1300 barrel and have gas ports added and that should work.
no you cannot. sabot slugs are made for rifled barrels. I would buy a rifled slug barrel.
A 12ga Winchester rifled Foster slug averages .713" in diameter
That is correct.
Yes, I don't think you should have a problem, but your pattern may not be consistent.