i don't think it really matters left or right handed you just adapt to the circumstances
i am naturally right handed but lost most of it in an industrial accident back in 1982, i basically have a right thumb left but it doesn't stop me pressing the shutter button with it
where focus shifting etc is concerned i have to pre determine what i want in focus and adjust accordingly before lining up the shot.
there's quite a few buttons on the 1dx 2 i don't use because i can't reach them but non of it frustrates me or stops me enjoying my hobby