I will say first is that I am no expert. There are many here much more knowledgeable than I am. I will try to answer questions as best as I can. Several of the members are at a "retreat" and may be off the site for a few days.
1. All uppers are not the same. It would be best to ask the maker of the lower what will function with their product. There are threads that point to the basic difference between DPMS and Armalite based lowers.
2. That is true to my knowledge. Others may have better knowledge.
3. Fire control group is not specific for AR15 or 308 for the most part. Some trigger manufacturers do have different models though. LPK are not all the same either. Some of the differences in manufacturers of lowers make it so that some pieces of a complete parts kit work and others don't. Again, asking the manufacturer of the lower may be of help.
4. Receiver extensions (buffer tubes) for 308ARs come in 3 common types: rifle length, DPMS carbine length which is the same as AR15 carbine length at 7 inches internal depth, and Armalite carbine length at 7 5/8 inches internal depth. Rifle length require rifle stocks and any carbine stock will fit either carbine tube.
5. Rifle buffers for 308ARs should all be the same length. Carbine buffers come in 2 lengths depending on which receiver extension you have. A DPMS based extension requires a 2.5 inch buffer. An Armalite based extension requires a 3.25 inch buffer.
A good reference for buffers, springs and tubes is here.
Or here. http://heavybuffers.com/
A good way to be sure 4 and 5 work together is to buy this. If it is OOS, call them or get on their notification email.
https://www.armalite.com/product/ar10rekit01-6-position-receiver-extension-kit/