Unfortunately the history of the diver does not necessarily qualify them to make the best decisions with current practices as their training may be quite old. There is no required con-ed in the diving world so while they may have been sidemounting for 30+ years, if they are still sidemount diving like we were even 10 years ago they are unfortunately very far out of touch with the current best practices and is actually where a freshly minted instructor who has trained recently on that type of equipment will actually likely be a better source of information for gear configuration. Many of us that have been diving for a long time have found what works for us and our specific style of diving and unless gear is being given to us for prototyping or as an ambassador then we are not likely to be familiar with it.
I still personally dive a first gen Dive Rite Nomad, and while I have some more modern style rigs, that still works best for the type of sidemounting that I do and will be the first to tell you that I am not the one you should come to if you want specific nuances of fitting an xDeep as mine is one of the OG versions and I'm not going to "stay current" with the gear trends unless they do something specific to improve my diving, and most of the other instructors are no different. We should all be well versed in the type of gear coming in, but well versed is a far cry from an expert.
It unfortunately sounds like this unnamed instructor is not giving you the best configuration examples for that type of diving and your specific gear configuration and sounds like a general lack of familiarity with it. We have been diving stages on xDeep style rigs for a very long time now and there are plenty of ways to skin that cat and I'm not sure that moving the attachment to the spine is a good idea. From a pure physics standpoint it is going to change the way the tanks hang and not in a favorable way, it can also be a potential hazard from an entanglement perspective in truly tight cave passages since you have something loose on your back and depending on your exposure suit you may not be able to reach it to detach, this is a huge safety concern in tight passages and it sounds like your instructor is not actively diving those types of passages with that configuration or it would not have been recommended to you.
My recommendation would be to use sliding d-rings for the 85's and that way you can slide them forward when you want to roll upside down. I certainly dive upside down on a regular basis though I would not recommend crawling on the ceiling like spider man. I can't think of any common high flow passages where that is better than properly reading the flow and moving along the walls. The sliding d-rings will leave you adequate space to slide them back towards the spine if you are diving with front mounted stages, though I will also recommend that you not move past 2x stages with sidemount as it gets to be very clunky. If you must, then you should be doing setup dives before hand where cleanup can be done with a leash to tow them out between your legs.
