1. A 14" barrel is a NFA item that requires Federal approval as a short barreled rifle, with extra paperwork and fees involved. There is no such restriction for a 16" barrel.
The specifications for your service rifle and ammo were very tightly controlled to assure reliability and accuracy. There are no such spec controls for commercial-built rifles. That said, put your money into a quality barrel, ammo and magazines and you will get the reliability and accuracy you seek.
2. There are many expensive boutique uppers available on the commercial market, but they don't perform any better than run-of-the-mill uppers.
Bravo Company has a good reputation for quality products and excellent customer service if you have a problem.
3. The easiest caliber change is to purchase a complete 6.8 SPC upper and magazines and swap them to your existing lower.
You are correct that 308 is not swappable because it requires a larger upper, lower and magazine due to the larger size of the cartridge.
4. The commercial market has many good options for triggers. The best are by Geissele in adjustable (DMR) and non-adjustable (SSA) models.