1. Are the barrels damascus barrels?

very good question, probably yes, we need pics

2. Will the shotgun barrels take modern day 16gauge 2 1/2" shells?

maybe, maybe for some rounds but maybe not, there is no 16 ga nitro load on the market somewhere on this planet with the recommendation from the maker for old blackpowder guns. I must say that there is nothing like a low pressure 16 ga cartridge available. I dont know what for ammo or reloading components are on the south african market. loading shotshell cartridges isnt so simply as it seems. if you can get 16 ga blackpowder ammo - I believe Gamebore in great britain load it - you can shot it without problems in your drilling. if not start loading your own blackpowder ammo and get a feeling for this stuff. I would allways use magtech brass but if you have plastic or paper shells cut it to 65 mm if its 70mm long.
think you will find low pressure nitro loads here but be careful because this is not for beginners http://www.doublegunshop.com/forums/ubbthreads.php?ubb=postlist&Board=1&page=1

and here
http://www.16ga.com/forum/viewforum.php?f=2

3. I have a box of 9.3 x72m ammo - can i use these instead of the 83mm rounds?

probably yes, not exactly like 38 special in 357 mag but similar

be sure you know your caliber, make a chamber cast and slug the barrel. the correct designation for this cartridge is 9,3x82R 360. better you get 82mm long brass, believe Bertram made it and Horneber in germany . the 193 grains 9,3mm/.364 softpoint bullet from RWS or Sellier&Bellot is the right thing for you. the 9,3x82R was becoming obsolete with nitro for black loads for the 9,3x72R because the 1 gramm blackpowder more you can get into the longer case could not match the nitro velocity anymore.
than there was also a NfB load for the 9,3x82R with the same 193 grains softpoint bullet in the same class like the 9,3x72R.

9,3x72R V0 590m/sec
9,3x82R V0 620 m/sec

9,3x72R loads will be a starting point for you. shot it over a chrony and increase the load till you clock at 620 m/sec