Having blocked a total of nine IP addresses and implemented the already mentioned defensive measures, user usage has returned to normal. The crisis appears to be over.
Now aware of the potential for further crisis a daily usage check will be made and corrective action taken when necessary. It is a small price to pay for security — this despite the enormous amount of time recently spent to understand and correct the problem.
Unfortunately, the mal-functioning, opt-in/opt-out, checkbox for tracking cookies has still not been resolved, but a solution may be forthcoming — a re-installation and relocation of the Matomo software. Unfortunately, this, too, will be very costly in terms of time, and I am reluctant to proceed until I am sure that it is the cause.