Re: using a bluetooth mouse on Galaxy S5. Mouse does not orient properly in landscape mode.
Think about it. Turn a desktop monitor on its side. How would you get the mouse to "orient properly"? You wouldn't .
Except you would...by selecting the monitor in display settings and choosing "portrait" orientation.
Not only has reorienting the mouse/screen for sideways(portrait)monitors been a standard check box feature in every desktop OS for over 20 years it is a common practice in office environments, especially in the software developer world where the ability to see more lines of text at once is helpful. Furthermore, hybrid orientations have long been supported as well. I'm using a Portrait, landscape, Portrait setup with my 3 monitor set up. The mouse transitions seamlessly between them.
To make things worse, using an otg mouse in landscape mode was possible in android beta 0.9 on the HTC G1(first android phone). So not only has mouse orientation adjustable in the desktop world it was historically possible on android as well.
The only truth to this whole post is that there is no built in way to achieve this functionality at present in android.
Not only was your desktop analogy rude and dismissive, it was factually incorrect at every conceivable level. I normally don't approve of necroing threads but I also don't approve of liars. People who speak authoritatively about things they know nothing about the way you do are in fact lying. Intent is irrelevant a lie was still the outcome.
If the op ever comes across this keep an eye out, i'll probably making a little app some time in the future to sync mouse orientation with screen. My guess is it will require root to accomplish but hopefully it'll be better than nothing.