Welcome to the forum. Feel free to post tech questions in the appropriate tech forum as you need. Check some of the stickies as well. I will try to get you started though.
1). The SPCII chamber is to the 6.8 what the 5.56 chamber is to the .223. It is a little longer in the throat area of the chamber to lower pressure. The cartridge dimensions are the same, but ammo loaded for SPCII (or the other improved chambers) may be too high pressure in a SAAMI chamber.
2). Stuck how, when removing the case from the die? Are you using Hornady OneShot? Try posting some details of the problem in the reloading section.
3). Lots of 18-20" barrel options. The 6.8 is a very efficient cartridge though and going longer doesn't earn you much more velocity. There are some 24" barrels, more 20" options, but 18" is considered the longest popular length.