How would you repair this copper drain pipe, and how much pipe would you replace?

There is a small crack in this drain pipe, and it's so thin there that I can bend it with my finger. The section that's palpably weak is no more than an inch long. The crack is exactly where the drop of water is in the top picture (just left of the Y, on the bottom of the pipe).

This section is hidden behind a finished wall in the basement. It runs into an unfinished area soon after and is visible from there too. There's about 30 feet of pipe in the unfinished area that is all fine on visual inspection and nowhere that I pushed on moves like the little bit where the crack is.

There is roughly 8 more feet behind the wall before it exits the basement to the street. I'm obviously not thrilled about taking out the entire wall to the room to replace pipe if it's in good shape like the sections in the unfinished area. Is there a chance the section that's leaking has seen increased wear because of the Y junction just before it? If so, it seems reasonable to repair it and leave the rest as is. Is that a good call or should I replace more than that?

What is a good way to patch this section given that I don't have much space for access without ripping out studs in the wall in front of it? The piece of wood above the pipe is a support for the stud that drywall is attached to in the room.