Questions
Outfits are stored in your account wide Wardrobe, so while any character on that account can use it, if you have a second GW2 account, any characters on that account won’t be able to access it.
I’ve never used any outfits myself, so I’m not sure if they can be unapplied. (They are free to apply.) Someone else here can probably answer that question.
It’s a checkbox to apply them over your current armor once you choose which outfit to equip. It’s off your Hero screen, equipment tab (first) and in there the outfit tab.
http://wiki.guildwars2.com/wiki/File:September_2014_Feature_Pack_Outfits_Panel.jpg
The one you select will then be in the lone outfit fix to the upper left of your character, with the checkbox enabling it or not. Now as for disabling the outfits helm/hat/mask, you will need to uncheck your head piece on your equipment screen.
RIP City of Heroes