Welcome to TFL!!!
Remember, just because the left-hand 870s don't come with a short doesn't mean you can buy a short barrel and put it on the 870. You could also have a gunsmith cut the barrel down too!
Mossberg shotguns are ambidexterous, they the safety located on top of the reciever so the shotgun can be used left or right hand with no problems. Mossberg also has a persuader line of defensive shotguns!