No, that's not necessary.
The last mission for the Imperial side of the civil war questline, is to assault and capture Windhelm, and kill Ulfric Stormcloak.
You have to go through the Dark Brotherhood questline. Innocence, my brother.
Kill a Stormcloak officer.
No. I was a stormcloak and wanted to join imperials. so i went to general Tullius and I couldn't join. All he says is that he wants whiterun back :/
It's a glitch, hopefully Bethesda will fix it :)
No, the Civil War quest line will give you your answer. Ulfric Stormcloak is believed to be High King, if you win the Stormcloak quest line, but Ulfric calls upon the Jarls to decide, so no one knows who inherits the throne.
Yes, you can.
You need to know the Clear Skies shout. Use it on the barrier and it will disappear. The shout is given to you as part of the Main Questline, so if you don't have it yet you have to continue the main questline until you get it.
No. When you have completed the quests, there is no way of getting it back!
Yes, you become a Nightingale close to the end of the Thieves Guild questline.
There are 15 quests in the stormcloak storyline, two of them give you an achievement, four are optional, one is missable and the rest are basic quests.
Couriers only come when they have messages to give you. There are messages for locations of word walls, inheritance from deceased friends, and important quests.