If an unanticipated problem occurs (unknown-unknown), should the devised workaround need to pass by the Perform Integrated Change Control process before it is allowed to be implemented?
Yes. The workaround created a change in something, whether it is a process change, a procedure change, a role change in terms of task assignment, a possible change in a needed piece of data, a possible change in output. The point of change control is to understand what changed, who was impacted, agreement on the change, agreement on a change in costs and risks, etc. The trigger of the change is a bit irrelevant, though needs to be communicated to those who would eventually approve the change.