I've seen monitor stands similar to yours with 3 (or more) arms. Designed to hold multiple monitors you should be able to find one that would essentially replace the one shown, be moveable, etc.
Like any stand you would want to watch the overall weight limitations but that said you could mount a speaker pair instead of additional monitors.
You'd gain in that its already aimed/leaning towards you. You wouldn't need them to be adjusted too much or too often.
Just an idea.

M2's might be big for that location size wise but would still be my first speaker of choice in this case.

I wouldn't have any reservations trying a pair of axioms PC speakers either. The first axiom speakers I ever heard happens to be the smallest I now own. It impressed me and personally I'd be excited to try them if I had a similar layout.

I'm not sure I'd install behind unless you go with a surround type setup.

In ceiling maybe.