1) It depends on the form that the Null takes. They are able to take 'protective' forms that hide the processor in the middle, but generally it will be visible, since they only have so much material to work with and they often form arms, legs, or other appendages. Presumably once a character has seen the processor once, they'll recognize it again or be able to figure out that (if the Null is in a protective form) it is in the central area.

2) So long as the Null is not protecting the processor with extra armor, a bullet from a gun would be able to penetrate it.

3) When we gave approval for firearms, we were thinking of basic guns like shotguns and pistols - the sort of things able to be assembled from materials in Hadriel, which are extremely limited. Unfortunately, that does not extent to armor-piercing ammo or an RPG, both of which require materials and manufacturing that are not currently available in the city.