Bolivar,
It sounds like you have avoided the problem, not found a solution. Which may be good enough for you. After reading the entire thread, my curiosity is piqued.
Simple solutions first. Have you tried a different Batch of the R 15? or tried that same batch of R 15 in a different load in a different gun to see if it performs as expected?
I don't think there is anything wrong with your extractor. It is doing its job. The damaged case rims are a result of the cases themselves holding on to the chambers tighter and longer than they should. The Brass is the weak link in the system by design, and is supposed to be the first one to fail. Since the extraction fails almost immediately when the rim rips off allowing the extractor to let go, the FTE is a symptom of an EARLIER cause. Tinkering with the extractor will only hide or aggrevate the problem.
The original riddle of why mag feeding causes problems, but not single loading is interesting to me.
If the ONLY variable that reveals the problem is the feeding, then the solution must address it before before or during the feeding process. Everything afterwards, like overgassing, is likely just a symptom.
I don't think a feeding problem should drive one to tinker with the gas port. Like the extractor, it works fine with other ammo. The relationship between this particular load and gun is the problem. Its far easier, cheaper and faster to tinker with the ammo.
If you are single loading with a Mag fitted, then clearly the resistance on the mag or the magazine itself is not the problem.
The difference in cycle speed between chambering a loose round and having to push a round out of a mag is significant, but if you used the forward assist to ensure it is properly seated, than its hard to see how that could matter.
Is there any chance that your rounds are being subjected to too much recoil in the magazine? You did a test to determine if chambering sets back the bullets, but what about recoil? Try this. Make up about 20 loads that you know to be of a safe pressure. Load three rounds into a mag and fire one. The second should get picked up by the action. Remove the mag and load another on top before firing again. Do this 10 to 15 times. This way the round at the bottom of the mag will stay at the bottom and after its been subjected to 10 or 15 recoils measure to see if its been set back. That recoil effect is cumulative, and is only a matter of time before it causes an overpressure. You know your crimp is fine when you are using 10 round mags and it takes MORE than 10 firings for the recoil to affect the rounds. Ideally 100 recoils would not move your bullets, but thats an expensive test.
Or Alternatively, try loading just one round into the mag and firing. Do this for your entire work up test. If you get past the grain count where the problem happened before, It could be the recoil.
You mentioned that once you had one FTE, the next round in the mag would also FTE.
QUESTION. Would you then to try re-fire that round which was rammed into the unejected case?
You tested your crimp with dummy rounds to see if the crimp was tight enough, but that was with an empty chamber. Do you have any idea how much additional force is applied to that bullet when its travel into the chamber is stopped two full inches early. Try that doing that test. I think you'll find that your bullets will be seated deeper. This does not, however, explain why the first one failed to extract.
Try measuring the diameter of the case at the mouth after its been fired. Comparing the stuck cases to the cases that ejected properly may reveal another clue. I can't remember if you said you'd measured them for length or not? any difference?
Lastly, I just checked my relative burn rate chart, and I suspect that R 17 is too slow a burning powder for the 68 gr bullets. This may explain why the 3031 works better, and why Hornady doesn't give data for it.
The issues already addressed by everyone may just be what happens when you use a powder that is too slow.
.....but my practical side says shut up and shoot 3031 or something else. I'm going to go with door number 2 on this one. 
