Water – Replace rotten bottom plate

Yeah, major mistake from the previous remodel with the the plastic over the insulation. All that does is hold moisture in and create mold. Your XPS and treated lumber sounds like a good plan. The plaster idea is also probably a good final finish idea.

Once you get the framing removed you should see the rough surface of the footing. It may look like pebbles but that should be the unfinished top of the concrete footing. Your wall is not load bearing just cosmetic so if it sits on the rough footing or the basement floor it should be fine.

The perimeter drain tile you spoke of is supposed to be on the outside of the wall at the footing and carry the water away from the house or into a sump to be pumped away.

Good luck!

The purpose of the metal tie specified in R602.6.1 is to ensure lateral distribution of the vertical load born by the top plate. Non-bearing walls have no vertical load on the top plate and therefore no metal tie is required in this chapter. You would still need to meet the requirements to protect piping, wiring, or equipment (.e.g, M1308.2 and P2603.2.1).

I haven't found a good discussion of a centered hole in a top plate. I would argue that IRC Chapter 6 requires only one metal tie under any circumstance and if the hole is centered then it doesn't matter which side you install it on. But again, if we are talking 2x4 framing then pipe/wire protection may drive a requirement for metal ties on both sides of an interior wall.

Regarding double top-plates see IRC Interpretation No. 15-03

Q1: Where a wood stud wall is capped with a double top plate, is the 1.5-inch metal tie as described required on both of the top plates?

A1: No. Where a double top plate is used, a single 1.5-inch metal tie is required on the uppermost plate only. No tie is required at the notched lower plate. This is illustrated in Figure R602.3(2) and Figure R602.3.1.

Simpson has a good reference for Intallation of Utilities in Wood Frame Construction. Combines structural and nail protection requirements for residential and commercial codes. If you are framing with 2x4's it is likely that the pipe protection requirements will push you to use PSPN516 (load bearing) or PSPN58 (non-load bearing) rather than one of the RPS repair straps.