I am going to reframe the Existentialism as Existential Psychology, because I am trained in psychology and understand Yalom better than Sartre. I feel that Sartre is a big part of modern Existential Psychology, even if they feel very different. Besides that the tone of the question seems to be looking for a psychological approach over a reasoned ethical one. But I am sorry if this is shaped wrong for you, and the whole thing seems alien to its origins.
Inauthenticity is 'characterized' in that way only in the sense that those are its symptoms, not its nature. And this is only the most common event of inauthenticity. When we 'split' over the importance of something to our selves, we experience a range of attitudes toward it all at once, and the two most likely attitudes to choose to express are the two extremes of absolute detachment and absolute attachment.
But for those two things to be errors is only a trend. There are times when real detachment is what you are authentically feeling -- when you really cannot affect a situation and have no empathic drive to do so. And there are times when you are in the unusual position of being able to capture something important in a way that is purely satisfying. People do that. And believing you might is not necessarily an error. (Just most likely. Especially if you feel that way all the time.)
You refer to the frustration of authenticity, but it is the substance of inauthenticity that is frustrating, no? It is when we find ourselves betraying our deeper sense, having different emotional directions thwart one another, for instance, and having to choose one or the other to surface and act upon, when both are, at core, equally ourselves. The frustration is the result of the idea that these internal forces must be resolved simply because the action they require must be taken.
We are bound to inauthenticity by an obligation to maintain narrative, so that our lives constitute a story and not a pure experience that has no hope of being shared. This is part of the way in which Hell is other people. We are tortured by what we think of as internal forces, or by what we imagine as introjected opinions of others, but in fact it is the compromise between our real experience and the narrative we attempt to make of our lives that is the torture.
From a more honest perspective, not every aspect of our lives needs to be encoded in a way that can be evaluated or shared. We do not need to live a narrative. And our realized actions and postures (essence) need not be what constitutes our own being (existence). This goes all the way back to Sextus Empiricus -- he escapes inauthentic commitment by 'bracketing reality' -- acting, but remaining uncommitted to the action taken, and waiting for our judgement to solidify. Therapists sometimes talk about this as having empathy for ourselves across time -- allowing that our actions are often just wrong, but that a better approximation to our real intentions was not clear or available at the time.
The problem with this is that we are then thrown into another temptation to inauthenticity. Time moves on, and we are taken with it, the moment of decision is past, and we are tempted to think that we must detach from it, ore return to it, even though we remain basically undecided, so it would be ultimately unwise to do either, and it would just make us feel neurotic. We can neither live in the past, nor simply suppress our own voice. Bizarre compromises like giving the suppressed voice its own artificial narrative in writing are common as therapeutic techniques, but in real life, they seem dishonest.
In fact, to maintain authenticity over time, one needs to cultivate the ability to maintain access to the attitudes upon which you have not acted in the past, without making of them some controlling artifact. We need not to shear the edges off of existence to preserve essence. (Or if we must shear them off, we need to keep the clippings for later use, and not allow them to collect and moulder.) But we should also eventually make peace with the unaddressed material. A large part of it remains unaddressed because it is spurious, or just plain wrong.
