In the interest of moving forward I tried to more seriously consider Dmitry's approach. Based on es-discussion discussion https://esdiscuss.org/topic/will-any-new-features-be-tied-to-constructors it seems likely new JavaScript features (such as private state) will be tied to object creation. This makes the prototype-swizzling design even less appealing, in my opinion.
Meanwhile, I have not made much progress on the cloning question. As Domenic pointed out, that would also either require prototype-swizzling or invoking the constructor, there's not really a third way. I guess for that to work cloneNode() and various editing operations would have to become resilient against JavaScript executing in the middle of them, something that has caused (is causing?) us a ton of headaches with mutation events. (Or the alternative, have some kind of mode switch for the DOM which is similarly a large undertaking.) Not sure what to do now :-/ -- https://annevankesteren.nl/
