Concerning the tantrum spiral, this is the information that I've gleaned from Legends mode, combined with my memories of the event (which are vague, trust me - I never expected things to come this far).
Some background - in 129, the Scorpion of Recreations (goblin civ) declared war upon the Diamond Cloisters at Weatherwires. The conflict came to be known as the Molten-Conflict of Combating. The first attack, made in the late spring of that year, was known as the Disemboweled Onslaught.
In the late autumn of 135 (shortly after the dome had been planned out and designated, and the top layers having been dug out), the Scorpion of Recreations attacked the Diamond Cloisters at Weatherwires in an attack known as the Riddled Assault. Seven dwarves perished in the initial attack - only seven. It was these deaths that set off the tantrum spiral. At this point, if I recall correctly, the fortress population was a little over 200.
Before winter arrived, the dwarf Stukos Girderopen went berserk and was slain by Udil Rimlulls the Purged Contingency with Fallengrasp the Poetic Illnesses (a masterwork silver warhammer). In an attempt to avert the already occurring tantrum spiral, I had a broad area of the fortress engraved. It did nothing.
By the time the new year had arrived, 61 dwarves had died as a direct result of infighting during the tantrum spiral. 54 had died of thirst or starvation after being stricken with melancholy, or driven mad. A well-timed (on their part) goblin ambush caused the deaths of an additional 3 dwarves.
Spring arrives, and the chaos continues. 14 more dwarves die at the hands of the angry or insane. 24 more wither away after refusing food or drink. A few ghosts rise, but are put to rest as quickly as those who are still sane can manage.
During the summer of 136, 3 dwarves die of thirst, and 1 of infection from untreated wounds (inflicted by another dwarf, of course). In the autumn, 1 more dies of thirst. At this point, the tantrum spiral is officially over. Total death count: 158 - included is the duke, Kogsak Murdershot. 168, if you include the goblin attacks that triggered the event and took place during.
You can see why I've referred to this as the "great tantrum spiral" ... about 5/6 of the fortress died in the span of less than a year. At this point, I might have abandoned - I have, in the past, when something like this wiped out the majority of the population. But I didn't - I had already spent an absurd amount of time painstakingly designating the outline of the dome. I used no utilities in the process - the entire project was designed and executed using nothing but geometry, a little algebra, and a calculator. I wasn't about to let that effort go to waste.
As for the dome... that depends on what you mean by 'finished.' The first stage of excavation was completed in the early spring of 146. Magma flooding took place from the late summer of 149 to 150. By 164, all accessible stone had been dumped, and the upper areas of the dome made contiguous with the upper cavern layer (prior to this, there had been a thin sheet of rock separating the excavated area and the cavern itself). By 180, all the dwarves had moved into the dome, abandoning the upper fortress - this is the stage at which I would consider the dome complete.
If you have any other questions, feel free to ask, either here or in a PM. If you'd like, I'd be happy to provide you with a save that has access to Legends mode up to the year 185, as well as my personal notes, which span the years 146-207. I stopped writing in my fortress journal after that year, and instead started making the public updates in this thread a kind of journal.