Hi!
First of all thanks for the comment and questions, and apologies for not replying sooner. I wasn't notified (or missed the notification) for some reason. This is probably late and I'm sure you've come with your own solutions, but it would be remiss of me not to answer your questions now.
DC is target number. Use stats as a reference: 3 is something all pariahs can do, 18 is something very few pariahs can do. Upwards of that it gets into murky territory.
In practice I work out a target number based on the difficult and their relevant attribute, and let them know what it is so they can decide if they want to try something else instead or spend a hit die if they are unlikely to make the roll.
Which brings us to "makes narrative sense". Yes, an example would probably have been helpful! Think of HD as spending your inner reserves: that extra bit of effort to make the shot or make the save. Would it make narrative sense for someone to exhaust their hit dice while making a stealth check? That depends on your table, and what makes sense in your game.
What prevents players from abusing it is that hit dice are a limited resource: pariahs only start with one. remember: HP are regenerated by rolling hit dice (similar to 5e) and the pariah must rest to replenish them.
Lastly, thanks for the typo-catch: there have been three revisions to the original volume that went out at zinequest, and I'm still finding stuff, so sorry about that.
You are correct: "Characters may NOT attack anyone except those they're engaged in melee with"