Some people think that every NPDer has some kind of a fragile mask, whatever. No they don't, only coverts have fragile masks beneath which they anxiously tremble.
Overt NPDers will take you on, there's nothing fragile about them, they're not wearing a mask and if they are it's a mask of humility, a fragile one.
If a person with overt NPD actually has NPD, the difference is that instead of anxiously trembling they will angrily rage to push away any potential for inferiority.
I would suggest overt narcissists don't seek out competition to prove themselves, they seek out easy-win situations and when they do lose they use denial and projection to avoid seeing that loss. I think you're right that overt narcissists who are primarily NPD rarely wear masks.
I think both have an underlying fear but the overt narcissist is removed from that fear by both base-level and surface level defences, while the covert narcissist you describe has less luxury of obliviousness.
I think a greater level of self-awareness - for whatever reason, is actually one of the factors which contributes to a person developing a covert rather than overt form of NPD.
When you start looking at narcissists who are egocentric because they don't have empathy instead of narcissists who lack empathy because they are narcissists (and all of the causes that come with that), I think you start blurring the lines into psychopathy.