First of all, Grindelwad cast a stunning spell on Gregorvitch. Just so you know.
Second of all, the defeat/owner rule doesn't apply to all wands the same way. Have you ever wanted to know what "slightly springy flexibility" meant? It doesn't mean you can use your wand as a slinky. What a wand's flexibility means is how temperamental it is in terms of changing ownership. Everyone knows that 'the wand chooses the wizard' (it actually really bothers me when people quote this because they always do it in a 'dId YoU KNoW' kind of voice, as if I hadn't spent hours studying wandlore and literally nothing else HP related), which applies past the basic wand-giving 'ceremony' at Ollivanders. Once a wand is in a wizard's ownership, it belongs to that wizard. Depending on its flexibility, its tendency to switch with someone else changes. For example, the 'stiff' flexibility means that a wand will basically never change ownership, while the 'very springy' flexibility means that it's got the temperament of the Elder Wand.
Also, it isn't necessarily canon that the only way for a wand to change ownership is through combat. I find it likely that combat ownership changes are just the only things we see in Harry Potter because those books have an awful lot of magical combat. I mean, we see people use other people's wands to an extent if the original wizard gives it to them (Neville uses his father's wand, Draco uses his mother's), though that's canonically not a foolproof system. And to answer your question further, Elu, I doubt most of the teachers even knew enough about wandlore to know that winning in combat could potentially end a wizard's ownership of their wand.
TL;DR, wandlore is complicated. In my own opinion I imagine returning someone's wand willingly would certainly do the trick... but then again, wands are finicky. So who really knows for sure?