jill:
1/ it sounds to me like this will work, since the window is high enough.
2/ There being a big window in the wall, don't worry a lot about what is "possible structurally under a load-bearing exterior 4" wall" because someone before you has already done the work to get whatever structural reinforcing was necessary to allow the window in the first place. The question is best when reworded: the window have already been "framed" and you want to put some thingie between studs, under the window. A plumbing forum is not the place to ask this. When you open the wall, underneath the window you will see studs but know this: they are not the studs that hold the building up; they happen to be the same size and spacing as all the other studs everywhere else, but this was done in order to remain standardized, not to hold the roof up. Of course you may wish to have all this confirmed by a couple other people including real on-site visitors. The remodeling forum might be a good place to go to.
3/ To put a wall carrier under the window might mean that you move a stud over by an inch or two. No big deal, not at all. Under the window you could also replace them with smaller ones and have more room for some continuous insulation (because you want to reduce air, wind, cold from coming in). Take this to the remodeling forum.
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Plumbing.
The drain pipe "angle" is also no big deal.
One day you can post again with a link to a photo on the internet, to get tips where to cut and which fitting to buy to connect to the drain.
One of the numerous advantages of a wall hung toilet with an in-wall carrier is that you can have a vented drain instead of a long S trap drain. (b.t.w., all toilets are designed to work on S trap drain configurations, so it will be OK either way, i.e. with or without a real vent or an AAV vent.) The toilet wall carrier has a drain : you could connect it to a vented drain line (e.g. using a real vent or an AAV (search "AAV").) Just one thing to know. Good to know. When it comes time to connect the carrier, you could remind the plumber of this.
About the elongated bowl: people get used to the bowl size they use all the time, so I would urge you not give in to the impluse to try to find the largest bowl possible. Any reasonable size bowl will be fine, if it's larger than the toilet you currently have.
Hth
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Tile
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