How secure is the 'magnetic snap', would you trust it to keep your phone, camera while mountian biking, trail running?

The pocket will provide a snug fit for most smart phones -- and that's where I keep my phone. The magnetic closure is good, but it is possible that (with enough pressure) the clasp could open up and the phone could slip out. Especially if the bag were upside down. The magnetic closure is not as secure as a zipper or velcro closure. But, it is not likely that the phone could slip out -- especially since the bag lies on the front of the body and you can see what's going on.

Would it be alright to wear it around your left shoulder?

Left shoulder, it's more to the front, on your right ribs, easy to get in the pockets. Right shoulder, its more to the back, just behind your left arm, much less accessible, but more out of the way. Quite comfy, either way.