I'll try, but I am not an expert. I would suggest you contact Browning or one of the highly esteemed Browning gunsmiths with specific questions.
1.) The basic BT-99 was an affordable single barrel trap gun that became very popular very quickly. You could get it in several different fixed chokes, in 30 or 32 inch barrel length, (later 34" was added,) with regular or Monte Carlo stock. Later on, the newer BT-99s were offered with choke tubes.
2.) The BT-99 Plus came with a built in recoil reducer and an adjustable rib. I think the adjustable comb and buttplate were also standard on this model.
3.)The BT-99 AR was built for a specific retailer and came with an adjustable rib.
4.) The original BT-99s were reputed to shoot at about 60/40 and many shooters who liked the gun requested that Browning make a gun that shot higher, so Browning responded with a gun that shot considerably higher. These guns can be easily picked out because they had distinct letters in the serial numbers.
5.) The youth model has a cut down stock to fit youngsters and smaller lady shooters.
6.) As far as I know, Gracoil has always had to be added after market. I could be wrong about this, but The Graco company is an entirely different entity than Browning Arms and the two have never collaborated on a new gun.
7.)The BT-100 was manufactured to be a newer and improved version of the BT-99. It features a drop out trigger with simple adjustment capabilities, ported barrels, Invector Plus choke tubes, a higher rib. Some people say that Browning dropped the BT-100 and returned to the BT-99 due to popular demand. A knowledgeable friend told me that Browning stopped production of the BT-100 because they they nwere losing almost $1000 on the sale of each gun. These were made for only a couple of years back in the 1990s. Martinpicker