3) A has nothing to do with B. If quantum computers ever get to the point of being able to do something like that (probably not in our lifetimes) the entire encryption of BTC would be broken.
To piggyback on this, also would mean that multiple industries would be compromised since they rely on the same thing Bitcoin does. Meaning, a solution would've already been found or Bitcoin being broken is the last of our worries. However, as suggested we've got a ton of time to think about quantum resistant measures which probably won't happen for a long time. I'm not sure about our lifetime, but I imagine we'll be ahead of schedule on a solution, since there's already been some half baked solutions proposed already.