Every Arcterex item I have purchased, I cried over spending the $$ but never regretted it.

Have you thought about a "tump line"? I know few people use this nowadays, but with your back problem, maybe it would work. You would have to rig it yourself - I doubt any manufacturer sells this.

I have done years of technical backcountry climbing. There are features on specfic mountaineering packs that are useful. Some very good backpacking packs do not work as well. Particularly, you need a good way to carry a rope, ice axe, crampons. You need to read reviews written in the climbing magazines. It has been about 10 years since I was into climbing so I hesitate to recommend specific packs. My mountaineering packs are all 15 years old!

Given the weight of technical climbing gear (sometimes this is 30 pounds of gear!) I would not obsess over the actual weight of the pack. The incremental difference of a pound in pack weight is pretty insignificant. You simply are not going to get an UL tough mountaineering pack. If however you just are carrying climbing gear to base camp and the pack never is taken up the mountain (think climbing chimneys) then it may not have to be as "tough" as you think.

My "system" was my Arterex Khamsin day pack (2 pounds) lashed to my Kelty frame (2.5 pounds), with an oversized stuff sack on bottom for sleeping bag, pad and tent. I simply took off the day pack for the climb. You can simply place the day pack on the frame as if the frame were a person and then snap the sturnem strap and waist belt behind the external frame padding, and attach the sides of the day pack with the compression straps on the day pack. My Kelty had an extender bar to which I lashed the climbing rope.

I also have a protruding tail bone although it does not cause pain. You may be able to rig a hip pad that is really thick on both sides of the tailbone and leaves a gap for the tailbone. Take an old pack and duct-tape on extra padding on either side of the tailbone to see if this may work.