since it's only about 3 feet that needs to come out, you just cut out the one or two studs a foot off the floor. Then slice out the bottom plate, leaving a bit of room to the next studs. then you put in a new bottom plate and another layer of bottom plate overlapping the existing. Then you splice in a doubled up stud . all in all about 30 minutes of work. If an entire load bearing bottom plate needs to be replaced then you build a temporary wall to support the ceiling joists and then cut out the plate.

I'd probably tear it all out and start from scratch. But of course I'm a perfectionist and tend to get a little obsessive. And no, don't skimp on the waterproofing.....