The rifle primers took out the vertical with the smaller powder charge. My estimate for 6.6 with LRP was too much, that went over the launch pressure window and started stringing horizontally and yawing, so looks like 6.0 and LRP is the ticket for this combo. See how the primer change was the exact equivalent of 1.0 grain of powder in your case? The groups closed up and got round (hit a node). With too much powder and the rifle primer, you got horizontal stringing, a sure sign of having passed the node.
Loob Boogers. Ahh yes. Loob boogers on 100 yard targets, together with a marked increase in group size and lower ambient temperatures, is what got me started on the quest for a better lube all those years ago. I was using Felix lube and below 50°F, groups would open up considerably. When I noticed the lube flecks on the paper, a lightbulb went on in my head and I went back and added some Vaseline to the lube to soften it up and make it less adhesive. Results? Groups returned to their hot-weather state and the boogers disappeared.
Ben's Red uses a grease which is loaded with polybutene stickifier. This chemical is what makes bar and chain oil stringy like hot cheese. Too much of it is not good (as in lubing all the grooves of a Loverin bullet) makes the lube stick to the bullets too well and the lube stick to itself too well (cohesion) unless the pressure and velocity is pretty high. The fix would be what Dan suggested, lubing fewer grooves...or I would say try a different lube, something like 50/50 beeswax and Vaseline for your low-velocity loads. The gas checked bullets might like a different powder, too. Also remember that gas checked bullets at low velocity need very, very little lube and adding more than necessary to a low-pressure system usually is a detriment.