I’m now leaning towards a faulty camera module.
Results of safe mode boot with Camera+ module installed: Same; fuzzy camera image, unable to focus either automatically or manually.
Internally, before doing this test, I checked both ends of the ribbon cable, the other end of which had indeed become loose under the EM shield (removed by taking out a pair of Torx 2 bolts), in common with this thread: Be careful with the connectors when replacing a module - #17 by Lidwien
Reseating this rectified an issue with the flash not responding, but did nothing for the focus issue.
I’ve once again reinstalled the original camera and have contacted support, who I hope will be able to help me out.
Thanks for your assistance.