Your take on xylitol is out of date. You ignore its ability to inhibit bacterial adherence. Researchers in Brazil (Ferreira, da Silva et al, PMID 25483720) argue that xylitol may be a partial solution to our problem of antibiotic resistance.
While it does cause GI problems if too much is used that issue is indeed universal; everything is toxic in excess. It works in the GI tract exactly like sorbitol, which is the most common remedy prescribed by hospice care workers for constipation. The only problem is that one does learn to metabolize xylitol so you need increasing doses that make it harder to play with.
Xylose you also misunderstand. It is plant sugar; what glucose is to the animal kingdom xylose is to plants. It is also one of the sugars involved in our immune complexes–those sugars and sugar complexes tied on to the proteins on our cell surfaces that are used to signal the proper interactions and or adherence. It is not foreign and is not toxic in moderation.
You are fearful that the enzymatic reaction using nickel gets into the xylitol? The definition of an enzyme is something that promotes a reaction WITHOUT changing itself or becoming a part of the reaction. There is no nickel in xylitol.
Please don’t ignore the decades of research showing how, why and when best used xylitol prevents tooth decay. While it doesn’t heal a cavity it does prevent them and you need to emphasize that rather than rely on the judgment of someone writing before any of the dental research even began. And if you do really like the old wisdom why not accept the GRAS (generally recognized as safe) label that comes from the same time?