I'm a lefty and holster options are not as open for me.
I think your left hand options are more limited at the local gun store, but not if you are ordering on-line from the maker. Just about any maker can produce a left handed holster for you.
Are mag releases usually covered for Right handed shooters?
Traditionally, I don't think they have been, as the "old school" guys taught the administrative reload which would have you eject and then load a mag while the gun is holstered. That would require access to the mag release button.
Modern Service Weapons article
http://modernserviceweapons.com/?p=2139
Nearly all of the long established holster makers, Alessi, Rosen, Sparks, Kramer, Del Fatti, etc., either had no sweat shield on their holsters or left the mag release button exposed. Possibly to allow the administrative reload, or possibly to keep leather away from the mag release to avoid possible activation. I don't know why they are such, but they all work pretty well in that regard.
However, it seems since the introduction the "hybrid IWB" holster popularized by the Crossbreed Supertuck, the sweat shields have gotten bigger and many now cover the mag release.
I don't think an inadvertent mag release should be much of a concern, though I'll admit the Gen 4 Glock mag release button is pretty big and sits rather prominently from the frame. If that is what you have, that may be a problem, but they are reversible I believe, and it should allow you to avoid that problem.