It shouldn't matter but the lock should be relaxed for installation. Now it sounds like your trigger bar is too high or your sear lever is bent too low. Setting the trigger lowers the trigger bar and allows the sear lever to drop enough to catch the tumbler notches.
One possibility is that...
The sear's not engaging the tumbler properly. Most likely wear on the tumbler or sear. Should be a simple fix but I'd take it to Deer Creek and let them show you rather than trying to hone it yourself.
Some of the Spanish CVA/Maislin locks have a tiny adjustment screw for the sear (on the...
Laminated steel is just a less fancy Damascus barrel; you have the same issues with both. The real concern is not the pitting you see but the subsurface corrosion that you don't see. In any event it appears that you have BP only proof marks, probably pre 1893, and a tight 12 bore...
I like Minwax Mahogany for color and linseed oil for a stock finish. After staining, use a piece of heavy canvas or denim to rub a few drops of linseed oil into the wood every day for a week, every week for a month and every couple of weeks forever after. Use the same cloth until it's worn...