advice from construction guys

hey WakeWorlders!
I am installing interior french doors between the kids' den ( formal living room) and my office (dining room). The doors are the pre hung variety that require a rough opening of 50" by 82". I need to build the rough opening.
A couple of questions:
1) I need to extend a wall by 2.5'. I know that I need to cut out the carpet underneath the sole plate, but do I need to cut the sheet rock off of the ceiling and existing wall to hang the vertical support and the top plate?this little extension will then turn the corner for the wall that will actually hold the doors.

2) In order for the wall extension to eventually look like it was part of the original wall the whole time, what measurement do I make so that my sole plate is in the correct location. I compensate for a piece of 5/8 inch sheet rock, but do I allow for any more thickness for tape,bed,and texture?

3) are there any tricks or advice to creating a header?

4) one side of the rough opening will be an existing wall. should I frame that with 2x4's or just shim the french door unit right to the textured wall?

I know it's a bunch. I'm a decent do it yourselfer, just wanting some guidance

thanks in advance!