Do you know WHY you pay each month?
Because the game isn't finished after you play for 20 hours...
It keeps going and going and going.
As for your other questions, no you won't die permanently. You'll probably respawn back in town, minus a bit of XP.
Yes, like any online game, you can't pause it. That's the nature of online games.
No the game wont end when someone kills the main boss. He will respawn and everyone will get a chance to kill him.
People can come and go online as they please. Bioware has even said you can pull people into your instanced events while they're underway.
If you want to do an event a different way, you'll probably have to use a different character. You'll end up having a few of them.
The game is broken up into quests. No one quest will take you 30 hours (i'm assuming).
If you do die mid quest and have to do it again, you should only lose 10 minute-2 hours of your time. Believe me, if you're at all serious about playing an MMO, this IS NOT a large amount of time.
You'll make plenty of friends in game from various time zones, and there will always be randoms you can party up with for quests.
Maybe you should go try another MMO for a couple of months just to see how they work.