In my understanding, all nodes currently use the coordinator to determine what is confirmed.
The coordinator has been turned off a number of times in the past and the network functioned.
This is true, but it was turned off to preform snapshots and nothing new was considered confirmed until the coordinator came back online and decided what was.
Also node runners already have the option to ignore the coordinator if desired so I don't see any reason a fork would be required.
Do you have a source on that? People could independently program their nodes to ignore it, or look for another one if the current one was ever compromised. But unless a node follows the rules that 95% of the rest of the network does, it can be considered not working, and be ignored by it's neighbors when they find out it is not giving the same results that they are.
I do not currently believe there is a software switch to just decide if you want to follow it or not. Nodes that do not follow the constitution of the network can be considered wrong and will often times be ignored by healthy nodes that do follow the network's constitution.
Edited: Reformatted the last bit.