If so, the series 80 uses some extra parts as part of the firing pin safety. This consists of a plunger that is positioned in the slide back near the hammer end. The plunger "locks" the firing pin in place (preventing accidental discharge if the pistol were dropped, and momentum carried the FP forward against a chambered round). The frame has a couple extra pieces as well. There is a lower series 80 part (trigger bar lever)which is pivoted by the trigger bow moving backwards. This piece then contacts the upper series 80 part (plunger lever) which in turn is pivoted to push up on the plunger in the slide, allowing the FP to move. Go to www.m1911.org and click on "parts diagram". Reference #'s 10, 31 and 52 are the parts in question.
As for interchangeability, you will need a "shim" to put where the plunger lever would go, if you were to delete the series 80 items, in an effort to have the pistol function like a series 70, which did not have those parts. You cannot add series 80 parts to a series 70, unless you drilled the slide to accept the plunger, and milled the frame to accept the plunger lever. All other major components are interchangeable.
While the Defender (and, maybe the Pocket 9 or Pony's, I can check at work Monday since we got a couple) is the only current gun to have it, I think the Double Eagles also said Series 90.
Not sure on that last one.
I think it's just a designation for the gun being designed in teh 90's. Nothing major as an advancement. (unless perhaps the Seecamp spring system is a requisite for the 90 series)
it depends on the gun, all series 90 guns have something different. a Defender will use standard officer's parts for the frame, but the slide, barrel and recoil spring is different.
Otherwise, yeah, you can use parts for it.