N1: As decoration takes place in workshops, you ought to be able to decorate only "available" furniture. I've never bothered with decoration, though.
N2: I believe so, I haven't examined my goblinite much, but I would guess some melted stuff was decorated.
N3: If your civ is *dead* you won't get a monarch. However, due to a bug, most civs that ought to be dead refuse to die and stay in the "struggling" state, which is where one of your dorfs suddenly become a new monarch. In addition to this, you can only embark as a dead civ if ALL dwarven civs are dead (I've made a pair of DFHack scripts to push civs that should be dead over the brink during worldgen, and add dead playable civs (i.e. dwarven ones) to the list of civs available for embark respectively).
Also note that you can occasionally get saddled with a new monarch even if your civ is/was healthy at embark, if the old monarch croaks, in particular if that was the result of the sacking/conquest of the mountainhome leaving your site the civs most glorious remaining site. I've never heard of a monarch at embark, so I assume that's no longer the case.
N4: There are a few shell bearing creatures you can get, like giant snails, FBs/Titans, etc. You can also get them from elven caravans in the form of live (caged) shell bearing creatures, and with luck you can even get a breeding pair. Fortunately, shell moods are fairly rare, but they do happen, so giant snails are attacked when they appear unless turtles are available, in which case I assume I can start fishing if a shell mood strikes.
V1: I believe all cavern fish are of the vermin variety that is "innumerable", but I don't fish except for shells (I'm struggling with overproduction, not to keep my dorfs fed...).
V2: I haven't heard of it, but can't rule it out.
Strike the earth, and remember that when the first fortress fails, that's only to give you additional knowledge for the next one.
(And I assume you know about multi level trees, as you didn't ask about those).