Those guns don't have a Damascus finish, they are Damascus barreled guns. It's sounds like your gunsmith has limited knowledge about Damascus barrels. At this time I shoot 10 different Damascus barreled SxSs - Parkers, Remingtons, and a Lefever. Been doing it from around 2005 and in that time maybe another 30 or 40 guns with Damascus barrels. BP at times but 95% of the time with nitro loads. The British have always proofed and shot Damascus barreled guns. They never got the bad reputation they did over here. Sherman Bell a couple of years back tried to blow up in two different test, 40 " Damascus wall hangers " with Remington proof loads of 18,500psi and couldn't do it. I keep my reloads to under 9000psi out of respect for 100+ year old stocks.
To answer your question, BP has a much bigger range of powder and still be safe to shoot. I've loaded from 70grs of 3F to 105grs of 2F.That's in the 12ga with 7/8, 1oz, or 1 1/8oz loads. In the 10ga I like for claybirds, 80grs of 2F and 1oz of shot. An adjustable powder/shot measure is usually used. You set it for the dram load you want and both the shot and powder is measured with it without changing anything. Shot and powder, by volume, is used. These can be bought from Precision Reloading or Ballistic Products. You'll also want to use fiber wads, not plastic. They'll melt in the barrel leaving a mess to clean.
Forget the Magnafluxing. I've put on a SxS event since 2005 and about half the guys shoot Damascus barrel guns. I don't know of anyone who has had their barrels magnafluxed. New ammo of any kind is hard to find but there are low pressure, low recoil shells out there. Also target loads at 1200fps or less. Some of those dove loads are up around 1500fps and it takes more powder/pressure to get shot to travel that fast.
You could go to the Parker web site,
www.parkerguns.org, and in their reloading forum there's a treasure fold of information for tested nitro loads, or info on BP loads. Good luck.