Anything is possible if you spend enough money.
Polished
No, due to the damage steel does to the barrels. Not recommended by Browning
You risk damaging the barrels if you do.
Not in Belgium barrels
The steel is stronger, but that doesnt nescesarilly mean its better
350 USD
No
Yes, all Browning Invector and invector plus barrels can shoot steel shot.
No
This goes back to the beginning of the auto 5 production in 1903. John Cockerill was a British Industrialist whose company supplied high quality steel to Fabrique Nationale (FN) which was the maker of the A-5. The Cockerill name was first used on Browning barrels stamped "cockerill steel". The steel was a very strong quality that met the proofing requirements of guns that were shooting smokeless powder. Eventually the Cockerill name was dropped from the barrels and "special steel" was used instead. The FN guns that were not sold to Browning in the US, uses the barrel stamp of "acier special" which means special steel in the French language.
Made in 1961. Special steel refers to the barrels. Do not use steel shot in it.
In order to connect a chromed steel pipe to a copper pipe, you will need a brass fitting. These two pipes will not fit into each other otherwise.