Full-face helmets
 

1. Can full-face helmets have the face-plate opened in order to eat or get peripheral vision?
2. Can full-face helmets be worn without the additional “face” part?
3. If yes to #2, can a full-face helmet enchanted with +2 DR be worn without the face plate without losing the enchantement?
4. If no to #3, can a full-face helmet have the face part removed prior to enchanting for +2 DR, getting the bonus everywhere but the face part?
5. If no to #3 and yes to #4, is the cost to enchant the face plate on its own $400 again?

Re: Full-face helmets
 

Just get a completely enclosed helmet with no eye slits at all and cast invisibility on it.

Re: Full-face helmets
 

You probably should take the helm off to eat. But maybe not to quaff a potion. But sure, sounds reasonable and most real helmets with face protection do either open or have the face guard entirely separate.

I'd rule that was the borderline on the enchantment too. If it's intentionally removable, it's a separate piece for all purposes. If you enchant it separately it makes no difference to your other enchantments if you are wearing it or not, and indeed you could probably tie it on somehow and have it protect your face without the helmet protecting the rest of your head. If it's attached you can enchant it as a unit and removing it entirely does whatever taking off a piece does to an enchanted full suit (which I think may be nothing for DF, simplified rules after all). Though why would you? And sure, if it's enchanted as a unit and you flip the face shield up, it's not broken, it just no longer adds DR to your face. In principle I guess it would count as two layers of DR if somebody specifically targeted your forehead behind the flipped up plate for some reason, but again, simplified rules, I'd probably ignore that too - though maybe not - whatever convoluted reason somebody thought that was a good idea might require otherwise.

It's expensive ($10,000, plus a custom designed helm if you actually want no eye slits), and watch out for the inevitable No Mana Zones.