1- They come set from the factory, you really should not have to tweak them around all that much. If you get a little binding, make sure the bolt sled is nice and straight, and you should be in business.
2- You would have to give Mel a call, but I'm pretty sure it IS available as an option.
3- They do come with a rail standard, and CCM also offers a little micro drop forward/backward rail, which should help add some length if you so desire.
4- Provided you keep that area of the gun clean, you should have no issues. If you're really concerned with getting dirt in there, I recall there being a thread in this section about a debris guard that someone had whipped up.
5- I'll have to let someone else answer this one, I have not used one of the breech insert bodies. Too long for my liking.
6- Personally, I use the standard CCM barrels. They work great, plenty of sizers available, and they're reasonably priced. It may be cheaper to grab an autococker threaded freak back if you can find one, but the deadlywind is a VERY nice barrel. Very light, and since it takes freak inserts, it shoots phenominal. If you're not too concerned with it, save the $90 and grab some sizers from CCM.
Retiring from paintball, things for sale!