I would paper the ledges around the windows and paint the inside of your closets white. That way....whatever color your walls are, you closets will look clean and white.

I think I would turn the corner and paper to the edge of the metal window jamb.

As someone who has done alot of wallpapering over the years (probably too much!), I don't think I would turn the corner and paper the edges of the window because of condensation and cold weather (if that is applicable at all) as it will just become "unglued" over time.
I've always just done the walls (not wrapping the corners) near the windows. I always had some kind of window covering that covered the sides and top of the window, though, so it wasn't necessary to cover the inside of the window. That being said, if cold and condensation isn't an issue, I think it would look really nice to see it like that!

I would just paper the walla and paint the area adjacent to the window (sill, sides, top) in a color that closely matches the background of your paper. As for peeling, as long as you are using a good paste, I wouldn't worry. I have a number of door opening without trim in my papered kitchen (heavy traffic...kids, dog) and haven't had an issue. Caveat...had my paper hung by a Paperhanging Guild professional. As for the sliding closet doors, hmmm? I am assuming they are currently painted? Again, I would paint to match paper.

Here is the way I did a closet in my hallway...I also have trimless windows and doors in most of my house.

I wrapped the paper around the corners, after taking the doors off first, and cut a straight edge along the top, so the top is just painted to blend with the paper.

As for the window, I would just paper up to the edges, and paint the insides a coordinating color.

Sandy, that looks really nice.

i also would paper to the edge and paint the drywall around the window in a color that closely matches the paper...
just make sure there is enough paste to have a good seal to the wall on all the edges...

Thanks everyone! Sandy, your photo was a big help...just have to figure out logistics of where to begin this job...I've never papered aroom with so many "issues"!

Justsaying, thanks for reminding me about curtains--they'll protect the paper around the windows. And I hadn't thought of weather related window issues--wrapping paper into the window=bad idea!