If you mean the two barrels are separating from each other, then the solder joining them has failed. A good gunsmith can repair this. If you mean the barrels are "unraveling" then you must have an old twist-steel barrel that has been over-stressed and it cannot be repaired.
Focus on the target, ignore the barrels.
One or two, depending on how many barrels the shotgun has.
AFAIK, no.
both barrels are not choked
Store brand shotgun with two barrels, side by side.
Try e-gunparts.com
That would depend on the make and model of the shotgun in question,and the length of the barrel/barrels that came with the shotgun.
No
They probably denote what choke the barrel has.
Online
Open the barrels and look on the flat part of the watertable (the flat part of the frame where the barrels close up) or the bottom of the barrels for the serial number
Rifle, handgun and shotgun can all have "long" barrels