I have not read this entire thread, but one thing I would look at is the hammer on the RR trigger. I don't know how many times folks have called me with the same problem you are having and it turned out to be the hammer.
With the upper off of the lower, take the lower and push the hammer back all the way. Hold it down as far as you can hold it with your thumb. Measure from the top rails of the lower receiver to the highest part of the hammer, all while holding it down as far as it will go. Now, take the upper receiver, roll it over and measure the distance from the bottom rails of the upper to the center of the carrier, the flat run that the hammer rides on. If the two measurements are nearly the same, or if the hammer sticks up further than the carrier is recessed, then the hammer is pushing up on the carrier and causing excessive drag. IF, this is the case, and only IF this is the case, taking a little off of the highest part of the hammer. It normally only takes a few thousandths of an inch.
You can normally "feel" the carrier begin to drag as you pull back slowly on the charging handle. When the cam on the carrier hits the hammer and pushes it down, you can normally feel excessive drag if the hammer is too high.