xylitol's selling point seems to be "not as destructive as sugar" rather than positively preventing decay.

Research 'xylitol, mothers, teeth'. That should hopefully bring you to the study that was done on mothers who were given xylitol during pregnancy and/or the early period while nursing newborns. It was found to have an actual protective effect and, if I recall, delayed the spreading of 'nasty' bacteria to the child from the mother.

Xylitol also kills bacteria in vivo. In laymans terms 'the bacteria notice that it is sugar but don't realise it is weird alcohol sugar so they eat it then starve'. So it is not merely a way to not have sugar in your mouth while also getting your 'sweet' on. Note that it also kills 'good' bacteria in the same way so too much isn't recommended, for the sake of your digestive system!

As for garlic... sure, it kills bacteria, but really, a clove a day... that isn't one I've chosen to make a habit of. I'll use listerine thanks!

RE: Xylitol again. I do recommend chewing gum flavoured with the stuff, not necessarily actually eating it!