Hi,
1/ Yes, and no. There are differences in people. Some can dive with no conservatism, and some cannot dive with all the conservatism they can possibly muster. Where do we draw the line? Does the slowest 0.001% set the standard for everyone else? Or do we make the 0.001% face up to the reality?
You say the models are wrong? Really - by how much? When Dr. Buhlmann published, he had testing with it. I'm confident his work was good, and his calibrations valid. But no one much wants to do real ZHL Buhlmann dives profiles anymore.
What about aging factors? We do absolutely nothing about this formally in dive planning. It's completely ignored by the aging re-breather population, who are the main complainers and source of these issues of late. Yet, I have here in front of me, a new book by Mitchell and 3 of his colleagues on Diving and Sub-aquatic Medicine 5th ed. In this they make is clear the effects of aging and how it requires more deco time to compensate. The cutoff point they suggest to slow down is 4th decade.
2/ Are you familiar with the expression, "if some is good, then more must be better" ? I think it applies to deco too. The VPM-B model has the conservatism changes made integral to the model core calculations - conservatism is baked into the computed result. Meanwhile, the other models have no conservatism mechanism at all. In ZHL and other Haldane designs, your options are to recompute the plan with added fake depth, or added fake time. Or use GF to stretch out the result. The VPM-B conservatism is based on science and parameter changes, while the ZHL is based fudged on extra time. Enough said.
3/ The 20/85 setting is yesterday's "standard GF". Today's "standard GF" is 40/70. And tomorrow it will be something else again. And so it goes on, because new people find new favorite settings, that fit to the narrow scope of dive ranges they choose. The standard GF numbers is vague and roams around, because GF is not a model or a baseline - it's extra time padded onto the end of ZHL model result.