I did the testing with my heavier lens. The pano axis is about where my crude earlier measurements put it (near the back of the lens, in contrast to my 28-70 which has it near the front of the lens). As near as I can measure the weight balance, it is in the same place as the pano axis. The big volume of the front of the lens makes it look like the weight balance would be further forward. But it isn't.
Continuing to play with the new tripod (bad weather outside and no good subjects inside for more real use of it), I am finding some flaws. Not close to enough to make me wish I had gone with a significantly more expensive one, but flaws:
The legs have angle locks at just 3 angles, narrow, wide, and very wide. The narrow is a bit narrower than is comfortable for normal use, but is the angle lock I'll be stuck using most of the time. The wide is too wide to be comfortable for normal use. The very wide likely only makes sense when swapping out the center column for the mini center column and then using the tripod super low. I'll need some experience to get a feel for when it is safe to not lock the leg angles, just set any desired angle between the narrow and wide. In all cases (even a smooth floor) it feels safe. The feet have a lot of traction and don't slip. But I probably won't do that on hard surfaces. On carpet and outdoors on grass or dirt, I think the feet won't slip and getting a better angle will be better than using the lock. But I'll need to play with it more to be confident.
Either the shape of the central part is cast a bit off or one of the leg lock mechanisms is a bit off, because one of the legs locks a just tiny bit narrower angle than the other two. So on a perfectly level floor, with all legs the same length, the center column is a tiny bit off of vertical and the plane of rotation a tiny bit off of horizontal. For perfect panoramas, I'd need to adjust that (by setting that leg a fraction of an inch shorter) and there is no built in level below the ball for getting that perfect. Other than perfect panoramas, that whole detail would be invisible because tilt and elevation would be adjusted anyway with the ball. For panoramas under normal conditions (on a surface that isn't perfectly level) I'd be adjusting tripod leg length anyway to get the rotation plane level.
The head easily removes (such as for moving the head to the tiny substitute center column). BUT putting it back before learning its problem led to a difficult to correct situation and close to a disastrous problem that I still don't fully understand. A double ended screw screws both into the top of the column and into the bottom of the head and it seemed OK to do both at once. That caused the screw to go too deep into the head, leaving not enough threads to hold right in the column (that part I understand and now know how to avoid). It led to the head being able to jump when just slightly bumped from feeling totally tight to allowing massive wobble (and maybe fall out). Glad I'm learning all this before real use. Misunderstanding and trying to tighten better led to the whole thing stuck: wouldn't unscrew from either the head or the top of the column. Careful application of more force unscrewed the top of the column from the rest of the column (which isn't designed to come apart). Somehow that took the stress off the threads so that top bit could unscrew from the screw, and doing that took other stress off (understandably because a grip part acts like a lock nut) so the other side unscrewed easily from the head:
Bottom line (at least so far as I think I understand): First screw the top of the column tight to the column (loctite it if it come apart again), then in normal assemble first screw the double ended screw tight to that top piece. It can't go in too far. Last step screw the head onto the other side of the double screw. The only thing stopping it from going in too far is that it is done last.