Before I go into my setup, because it doesn’t match yours, let me just cut to it: experiment before drilling and mounting anything. Any advice you get will be subjective and cannot possibly factor in all the specifics of your room’s current configuration. Rig some stands out of whatever you can to temporarily hold the speakers in place or get two helpers to hold the speakers in position and then turn on a good movie action sequence and also compare that to a normal dialog sequence with little background noise and see what position works best.
I have a 5.1 system with a decent receiver that everything runs through. My two rear speakers are mounted on the vaulted portion of the ceiling angled down to point at our heads in the bed. Very few movies use the correct audio format and channel isolation necessary to produce noticeable sound out of the rear speakers. IMO, anything but DTS sucks and even then when a studio attempts to use DTS they rarely get it right but when they do it is mind blowing. Even better than the theatre. So for a 5.1 or higher speaker system I don’t see an issue with the speakers being higher than your head and angled down. This is what the theatre is doing anyway.
This pertains to your setup:
My receiver has a bunch of audio “modes” that can make the sound reverberate like in a theatre, stadium and so forth. What I can say after all these years is that if the voices don’t seem to be coming straight out of the TV I can’t stand it and I only use the audio mode that has ensures voices are coming only out of the center speaker. Since you have only two speakers this may be tough to achieve if you mount them on the walls next to your head. The only advice I can give is to re-iterate that you should experiment.