Connecting a partiton wall to an existing wall

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I am in the process of finishing my basement and want to add a partition wall to extend an existing wall along the stairs back to the back wall. We have a walk-out basement and that part of the back wall is above grade so it is conventional wood frame and is finished on the inside with drywall. Where the new wall would join the back wall is between the studs. Also there is a 5" septic pipe at the top ot the back wall that prevents me from both running the top plate all the way to the wall and running the ending stud all the way up to the floor joists.

Here are my questions:
1) How do I attach the new wall to the back wall? Do I have to open up the back wall to add blocking?
2) Is there a standard way to frame around the pipe? My thought is to end the top plate at the last stud, add a jack stud next to it and a stud of the same height at the end with a plate/heater running on top of them.
3) I would love to be able to hide the septic pipe somehow (like with a cornice) but it has a cleanout that I suspect I need to leave access to.