For the first part, they are probably the extended carapaces. They come with 3 pieces and are flat (1 with a hooked end though). The other sprue is for monsterous creatures. It also comes with the venom cannon and barbed strangler.The UK website has a similar pair of webpages.
They don't really help me though. One of the sprues has 6 toxin sacs labelled 3, 3 adrenal glands labelled 14, then then three something-elses labelled 4,5,6. I don't know what they are, or where they'd go.
There's another sprue with that has big scything talons and the rending claws which has things labelled 9 and 10, but again I don't know what they are or where they would go.
Do the Tyranid Warrior sprues come with all the different biomorphs or for some do you pay the cost, tell your opponents but have nothing to show for them on the figure?
As for the final part: Buy the codex. It would have most the answers you need, and would tell you that you need to pay for all the upgrades.