I haven`t many answers, but can try to tidy up your queries:
Are you proposing replacing the rear twin wheels with single wheels then?
Why is/was the vehicle tacho exempt? And does anything change that status?
If you tell us the exact vehicle and what you wish to do with it, you may get some more precise answers.
Wheel sizes are much more than the outside diameter and the number of studs.
You also need to consider the PCD of the stud holes and their diameter, the bore diameter, the offset, the width, the load rating etc. Ensure that the slightly larger tyres dont foul body, suspension, hoses, wiring etc. MoT test doesnt include spare wheels. If you have an unusual, hard to obtain, size/type/run flat, and go off road, then common sense would point to carrying a good spare of course.
MoT test for commercial vehicles does look at load ratings on tyres.
On the tyre sizes you`ve given there would be a speedo error of about 5%. The speedo would read slower than actual speed. Tacho calibrations do include tyre sizes.
Maybe this would be better discussed in the Owner thread? Those with spannering expertise tend to hang out there.