And I've gotten my 1.200.000 experience points, is it possible (read: "should I") to do the very last quests for both the umpani and t'rang, meaning destroying both their ships as well? I remember you get good xp from destroying those ships, and seeing as they won't meet you on asc. peak.. what's stopping me? (besides the bad conscience)

Well, say you destroy the t'rang ship first, you can get the XP from the Umpani general if you go see him before destroying the Umpani ship, but once you destroy his ship, the Umpani turn hostile. In other words, you can get the credit for completing only one of these two missions (other than the XP from the actual act of destruction).

actually. When you destroy the t'rang ship after doing all the quests for the t'rang, they don't turn hostile, they go neutral. Is it possible to speak with neutral npcs?