"put down rebellions,"

[Alneyan]

1) Your homeworld cannot go below Angry. If you are playing with more than one homeworld, the others might riot, or not. I am not too sure here.

2) Happiness is the main factor to determine production. Jubilant gives +20% to production, Indifferent gives nothing (which could be argued to be a loss compared to Jubilant), and Angry gives -20% to production. So going from Angry to Jubilant increases the production of this planet by no less than 50%, and going down from Jubilant to Angry decreases your total production by one third. And obviously, once you are Angry, the massive riotings are waiting for you.

Your happiness is, to the best of my knowledge (the happiness.txt file) not affected by glassing enemy planets or destroying Empires. Even if you have lost one of your planets, glassing it will decrease the happiness of the player currently holding the planet, and not yours. Capturing a planet increases your happiness, however (and lowers the happiness of the other Empire).