I think the first thing to realize is that there is never a answer to what if questions or technique vs technique, style vs style, concept vs concept type inquiry's.
On the street, you don't know what the fighting background is of your opponent. If I try and be passive/defensive in a effort to find out, then I will be dead in a quick minute. The idea is to win/overcome/destroy whomever is in front of you, so what do you do, or what does WC have to say about this? Well from my limited understanding, it says to be proactive, attack, always move forward, and hit, hit, and hit some more with all you can muster out of the body you have, with whatever weapons you have available to you on your body or that you can grab or that you can use from nature (tree, cliffs, man made structures, etc..). I don't care if he is a boxer and can bob and weave, or that he is a wrestler that is going to take me down. If you start worrying about those things then you have alread lost. Just do your thing, and hope the training/understanding that you have done in the past was enough to get you out of this situation alive and relatively unharmed.
You see there is a difference in training, as compared to application. Training is what we are talking about mostly around here. So when I chi sau, I am being specific in nature and working strictly on my "Wing Chun" habits, and not about fighting application, since the drill is far from a fighting nature. So yes, in chi sau there is a set of guidelines and if your partner goes out of them, then you are no longer doing chi sau. This is what you see mostly when people try to compete in chi sau comps, they start out rolling, and then lose all idea of what they are doing with the sole intention of hitting, in any and all ways, most of which is not WC related. The ego kicks in and people feel the need to do what they have to do to save face. The drill is not meant for that purpose but to teach specific things, in a specific way. All of WC is like this, but in application it is me that is fighting, not the art, and I choose what to use from it (by force of habit or unconsiousness, or by experience and wisdom in the art, all of this depends on time in, skill and experience using it in real situations, lots of variables to consider here
). So if I am out of position or taken by surprise and unable to raise my hands up, guess what, I may bob and weave, duck, etc, to avoid a blow, since the natural instinct is always to move the head. God help me that the Wing Chun gods don't shoot a bolt of lightening upon me when I do that, lol.
For some reason, people on forums are looking for absolute answers to questions that are related to an activity that is far from predictable. The fact of the matter is, no one is invincible, everyone is vulnerable, no Martial Art guarantee's 100% success in a fight, training hard and understanding what you are doing is very important, guidance from someone with experience and skill is important, and all we can do is increase our chances in a fight by practicing a Martial Art. Also the fact is that most of us train in things that we ENJOY doing as a activity, rather than purely for the fighting effectiveness it gives us. I love training in WC, that is why I do, not because I need to learn to be a effective fighter, since I haven't (like 99.9% of the rest of us on this forum) had a fight or chance for a fight in years.
"Avoidance is the best thing to do, and learning art of invisibility is the best thing to learn if you don't want anything to do with violent physical confrontations." WSL quote..
James