Oh beloved tantrum-spiral...
So, funny thing about the tantrum-spiral. It is entirely avoidable if you embark with a proficient chef/brewer. The bonuses from having a legendary dining room to happiness stacks with both food and drink separately. If you have dwarves who eat in style, have perfect quality food, and perfect quality drink, they're un-tantrum-able. And if they eat one of the "legendary" meals your dwarf inevitably makes, you could kill off their entire extended family and they'll stay happy for as long as it stays on their list of recent events.
And about early fortress deaths due to invasion: I find a pit spanned by a pair of max-length bridges, linked to a lever that retracts the bridge can serve quite well to contain any invading armies. Just funnel some water into the pit once you get around to it, let them drown, and drain the pit again later. Good old dwarf engineering.