I shoot a Beretta 686 UL 12 ga O/U, will answer your questions, and hope they are helpful for your situation.
1) No, I want the tight barrel on bottom and the more open barrel on top. I shoot a heavier load (1.625 oz TSS 8.5x9s) from tight choke, and 1.25 oz TSS 8.5x9s from the more open choke. My gun's POI is much closer between the two barrels so loaded. In most cases, the POI of your top barrel will be higher than that of the lower barrel, and that tendency is exacerbated with a heavy load in the upper barrel. The reason is that the upper barrel is higher above the bore axis of the gun, causing the gun to rotate about the bore axis upon recoil, throwing the pattern higher. Felt recoil is also lower from the lower barrel for the same reason, so that's where I want my heaviest load.
2) Can't remember the brand name, but I have a sling that has nylon webbing that fits around the butt stock with the other end threaded under the rib between two posts on the barrel.
3) Yes
4) What are you shooting? Shooting TSS, I shoot an extended Sumtoy .675 in the tight barrel and a flush Lt Mod in the upper barrel.
5) Yes, but as posted earlier, I would put the extended on bottom.
I have an FF III on mine on a B-Square cantilever rib mount. The red dot is regulated to throw the lower barrel a couple of inches low and the upper barrel a couple of inches high at 40 yds. I aim for the head with the lower barrel and for the waddles with the upper barrel. Perhaps you will get lucky and your POI will be identical for both barrels. Hope that helps a little.
