There are quite a few design patterns around that might be worth considering like the Dead Mans Switch mentioned on this post.
Thing is you need to consider which ones are right for you. For me I can only send ON and OFF to my physical light switch it’ll never tell me if someone has pressed the button.
Yes it is possible to set a state to UNDEF (or more precisely NULL) in a rule, don’t know about sitemap. A quick look around and you have to create a temporary item that will adopt the state of NULL which you can then pass to something like postUpdate. postUpdate/sendCommand both seem to protect themselves against being sent NULL direct! All seems a little dirty to me so I’d avoid using it myself, don’t know enough about where it is going long term.
I’d just implement the proxy as multi state if you want it to have the AUTO feature but I’ve no bindings to consider so it’s not right or wrong, maybe just give it a go with your UNDEF on one light and see if it works and your happy with it.