Obsessed, to answer your questions the answers appear to be as follows:

Yes, made as 12, now measure as 10

The chambers are 2 7/8 and measure for a 10 gauge shell

The proofs are unaltered I expect from when the gun was exported (consistent with Graham Greener's suggestion that this gun was made for export)

There is slight play between the barrels and action without the forend installed - according to my smith, the gun is on face and there is no issue with the hinge pin.