That's a bunch of questions all rolled together... I'll attempt an answer..
Yes.
No its not currently available.
Yes. (At least before Rem took over you could. If you still can remains to be seen.)
Again, Yes, at least you "ustacould".
Same stocks fit SB1/SB2 only exception would be the 10GA receivers.
Only time will tell.
I would guess about everything available now is from Gardener stock, nothing new yet. (Just a guess though)
Personally, I would look for an older rifle, You know Old/New store stock.
In the order you asked..
CW