Can Body number and build numbers be out by 10 eg 511 difference as opposed to 501?
yes, it's rare for them to be exactly 500 different, almost a reason to be suspicious as it's "too perfect". Mine is 510 numbers different
My car is more than 50 out.

Can engine number 558** be mid 1970?
my mk2 is 7/70 and engine number is 556**, 558** might be a little bit high, but the numbers weren't very linear
I have one listed with 9FXEY55810

What would 06 mean above build number on firewall behind brake/clutch master cylinders? 06 is the body code for a Cooper S. I think the 06 was usually behind the paint code sticker
It should read 2/06 on the MK2 S

Did MK2 S's have PBR plastic Brake/clutch master cylinders? What if a car had 1 plastic and 1 tin looking one? there was a changeover period, and stocks of the tin tank were replaced with the plastic PBR. You also have 45 years of maintenance that might've meant they were changed. Mine are both PBR
I personally believe that the clutch master cyl would have been changed over first as they were the same on all cars and would have been using them up faster. The Cooper S brake master cylinder was different to other minis as it was taller. It may have taken a few months before they used up all the stock on the shelf. By 1970 they would have both been plastic.