If the barrel has gone dark, then that is a metallurigical process and is not reversible, due to the formation of low-grade acids during the powder burn process. It is unsightly, but is not harmful. Some powders do this, and some do not. Yours did. None of mine did.
DO NOT LAP YOUR BARREL! Hege do this as part of the very low volume manufacturing process.
You'll have to call Hege to find out about the nipple - these pistols might be lying in every corner of the gunstore in merry ol' Finland, here in yUK but I've only ever seen one in my life, and that was at a gun show.
tac