I think you should take the Greener to a good gunsmith and ask his opinion, because chances are, after 100 years, the gun could be out of proof.
When a gun is re-proofed there is always some element of risk, but a gunsmith will advise you as to its chances of passing the test.
Before sending it to the proof house he will check that the action and barrels are jointed properly, that the ribs are sound and that the bores haven?t ?grown? in diameter beyond laid-down limits through either constant firing or overzealous cleaning, or both.
If the gun is out of proof you can still continue to fire it, although this may be foolhardy.
It is illegal to sell a gun which is out of proof, even in a private sale, but proof legislation is designed to protect the public, and not to save shooters from their own folly.