The gobbo did destroy the chain, and I think it was a tantrum destruction, so yes, it acted as a visitor from a mood perspective, but still as an invader in that any civilian seeing it (including the one chaining it up) runs away at top speed. The chain was a bronze one, so it should be too strong for a mere gobbo to break with force, even if that was possible.
The one necro I have chained doesn't scare anyone, nor does it raise corpses. A different one contained in an observation room failed to raise corpses a lot of times when mutually viewing a gobbo aggro observer, including when invader trolls were let into the areana to be butchered by the trap.
A necro released together with an invader troll in the arena with corpses present and a gobbo aggro observer did not raise any corpses (and the troll wasn't aggressive either).
Finally going back to the mutual observation situation, minus the trap which was removed before the troll/necro suspicions stare competition, suddenly resulted in raised troll parts, and I have no idea what's different.
Unfortunately, I let an ogre in to fight the troll parts, and the ogre has been unconscious most of the time for over a season without the troll parts managing to hurt it further. A save scummed attempt to clear out the mess with the "militia" (with gobbo and necro observers removed) resulted in 2-3 dead dorfs before the troll head finally was caved in (a brief look saw two obsidian short sword attacks pass right through the head).
I'm now considering pelting the head with bone bolts for a few years, alternatively doing the same with the ogre (expecting the "victorious" side to exit the arena into the weapon trap followed by cage traps).
(BTW, the ogres are LARGE: I was wondering why the elven caravan got severely overloaded (5000+) with essentially just goblinite clothing, and not much at that, and then found the ogre leather cloaks weighing in at 270 or so. That also explained why haulers were so sluggish just moving pieces of clothing...