If you can't clean up the painted edge, you could always add a 1x3 piece of wood as molding. You can even carry it up to the ceiling to frame that wallpaper. I don't see that paint color in the wallpaper, or maybe it's a lighter version of the flower centers. Maybe stencil on a design using the darker color, or another color from the wallpaper to help tie it together. Then narrow floating shelves or vertical frames or something in the painted areas it you feel you need it. Maybe turn the bed around so the wallpaper is the headboard.

Hi Ni, I just thought of something....did you say you planned to do the stencil motifs on the window wall? If so, I am thinking they will be mostly hidden by the curtains. I wonder if it might be better to stencil the long wall that is on the right side of the room when looking toward the window. I could see this being a pretty backdrop to the prayer box (I think that’s what you called it?) and your wood cuttings. Then maybe the opposite wall (the one with the kitchen doorway) could hold a large piece of art?

You definitely don't want to emphasize the T-wall, as it will just draw the eye to the mismatch in scale of door and window. I honestly am not sure that an accent wall does anything other than provide an excuse to make fewer art decisions, but I guess it could be ok on the larger wall...

There are lots of self adhesive wall papers with lot of good design n sizes u can stick them over it no need of professional help but try to get waterproof or the one u can wipe it with wet cloth some of the available on Amazon n cheap way of remodeling ur furniture look... I hope this can solve ur problem