Not necessarily - the software could be upgraded to take the existing chain, and upgrade it to a quantum resistant hash prior to any damage being done.
Yes, but then non-quantum devices will take forever to execute a trade.
If that's true, third party middlemen with quantum devices could work around this until it becomes more mainstream. At least it would allow it to not wither on the vine until everything is restructured. It's not really obvious what would happen in any case, but humanity has a way of papering over things.
Ok. I just wouldn't want to bear that risk and have a dime on any public-digitally-signed protocol. Let alone my savings.
If quantum chips work out then a ton of existing financial infrastructure using non-quantum-resilient security will also fail.
There is already a dependency between capabilities in trading (esp. high freq trading), things will adapt. Fear not.