Suggestion for rear speaker placement

1) It should work just fine. In-ceiling would also work there, if you did that, I'd suggest a model with a pivoting tweeter so you can fine-tune as you'd do with a speaker on a shelf or on a bracket.
2) if you mean angling down that shouldn't be necessary
3) I'd use a bipole or dipole speaker (or one of those that's switchable) there, that's personal preference, however. Some people like to try and make the rears very locatable (point source) whereas I always preferred a diffuse sound field behind for surround effects. A bipole will give you a big soundfield to the rear. If you like that "fog" of sound behind you, you might also try a conventional speaker and aim it at the back wall-- again, a preference.

Look at what's in movie theaters, and if you are planning on using that as your reference, even with discrete channels, you may not want a speaker that gives you a very localized sound to the rear.

Keep in mind, obviously your hearing sensitivity to the rear is greatly diminished in comparison to hearing forward in terms of your ability to discern tonal accuracy and locate sounds, so the rear speaker placement isn't the place to spend a bunch of money and kill yourself IMHO. Find speakers you love for the front end first--

Thanks for your advice. I should have mentioned that the back shelves are in a cabinet. I do not see how bipole or dipole speakers would work inside a cabinet.
I could simply place the conventional bookshelf speaker on the shelves?

yes you can place the speakers on the shelf and try to adjust it when you turn on the film until it sound please you,for HT Klipsch is very good.