I think that opening the wall is definitely in the very near future, yes. I'm presuming there may be a hairline crack in the exterior window sill that's allowing moisture to get in. For the short-term, is there anything I should do to protect against possible mould growth over the next two weeks while I'm away? My concern is for my cat.

Yes, I have done some research, explored the house more and I have started the prep work for re plastering. I was able to fly through the worst of the peeling paint in the old closet using a sharp paint scraper, and have feathered the edges for what is currently sticking. I am going to use a plaster weld/ cement bonding adhesive to ensure good adhesion for the patch work I will use a good sealing primer for the painting. I will just be diligent about removing the loose stuff. I will need to reinforce the attic in places as they used 1x6 backing for seams on the plasterboard, without proper blocking in places. I don't want more cracks when I go up there in the future. Found a condensation induced leak in the attic, leaking onto a covered roof. The list of work gets bigger and bigger. Just need to pace myself. I guess I just needed some encouragement and confirmation for the plan. Been watching this guy on youtube and have learned a lot about plastering. Not so intimidated anymore.

Marisa - I almost hate to add to this. But, I am compelled, since I "restored" a 1900 Victorian home. "Restored" rather than "Remuddled"! haha! So, here's the deal. I hear you. You do not have to repair the wall behind the trim unless the cracking and crumbling extends past the 3" trim you want. Otherwise, the quick and dirty method is caulk around the window and fill any big openings with insulation if you can then place the new trim around window edge, nail in and hope you don't get more cracking or you could use contractors glue with minimal nailing, paint and final caulk. Simple, and I am not understanding your GCs not getting this easy process since you aren't restoring the original wood trim. Have you saved the trim, btw? It will bring some cash if you put it on Craig's list or other sales system, unless your GC has already "taken it of your hands"! Good luck. I wouldn't try to fix any cracking until you are done, because the installation will just cause more. Really depends on the condition of the original walls. Hope you get what you are looking for!

Yes the moisture problem was over 20 years ago and it was not a burst pipe. The woman above us turned her sink on and then went to her bedroom and went to sleep and forgot about it. It's flooded and came down through the wall. This caused some minor swelling to the plaster and made the paint crack and peel. There is no sheetrock there what you see is beige paint underneath a layer of yellow paint. There is no existing moisture problem. It is simply a very ugly wall. I want to put something attractive that I can easily wipe down. Peel and stick seems like the best solution since it's easy to take away if I need to. If there is ever another leak or moisture problem the entire wall will have to be torn out and fixed by the owners of the property anyway. The amount of investment to do a peel and stick for this small area is $30. I am not about to go out and spend a stack of money on this wall when I don't own the property. A lot of this is really nice advice for someone who owned the property but that is not the case here. Surface is fairly smooth I was just worried that the areas of peeling paint would interfere with the adhesion of the peel and stick and was simply asking if I should put up some type of board first, smooth out the wall with putty or just go as is. That was really my only concern whether or not the peel and stick wood actually stick to this surface. Sorry if that wasn't clear in the original post