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 16 quests which are given to you by a daedric lord. One is actually part of the Thieves Guild questline.
You can't but you can get the Skeleton Key as part of Thieves Guild quests which is a unbreakable lockpick!
No, that's not necessary.
In terms of DPS, Chillrend would be, but only when obtained at lv. 81. Chillrend can be found in Mercer Frey's house during the Thieves Guild questline.
You have to go through the Dark Brotherhood questline. Innocence, my brother.
Yes, you can.
To enter Mercer Frey's house in Skyrim, you need to progress through the Thieves Guild questline. Specifically, complete the quest "The Pursuit," where you will follow Mercer to his home in Riften. Once you reach the house, you can use the front door, but be prepared for a confrontation, as Mercer will be waiting for you inside. Alternatively, you can sneak in through a side entrance after disabling the traps.
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.
Skyrim for Pimps - 2011 The Thieves Guild 5-9 was released on: USA: 14 August 2013
You can't without mods.
You can acquire the Skeleton Key through the console command player.additem 0003a070 1. Source: Elder Scrolls Wiki page "Skeleton Key (Skyrim)"