I would probably not allow a toughness power for a trenchcoat, since I would like to see a power like that reflected in the shifted form somehow. Scales on a dragon, thick hide on a bear, stuff like that. And in your description of a "Stealth and infiltration Specialist", there is probably a compel in there somewhere, too. I know, not exactly part of the problem, but I wanted to mention it anyway.
Now, despite the fact that your friend is actually wearing you as cloth, I would not make a difference between that and you standing in front of him, covering him with your body. You put up a block against him, using your body as a shield, you are not actually moving, so endurance would be the way to go. But instead of eliminating the stress, the stress that does not hit your friend will hit you. And I would probably compel you out of your form, once you took a consequence.