Thousands of cops and agents carry SIG pistols as left-handed shooters.
Decocking becomes a matter of figuring out what works best in your hands. Some folks use trigger finger (this is what I do when running WHO) but it often requires a little surrender of grip. Others use their left thumb, rolling it to that side temporarily.
Ditto with the slide release; I tend to use my left index finger when shooting WHO but that really forces me to let go of most of the gun.
Depending on your hand size, you may want to consider reversing the mag button.