About the guideposts: that's just not true.
Now let me first qualify my assertion. When a casual observer sees some object that clearly exposes an intentional manipulation of some material, is not evidently designed for utilitarian reasons, AND seems to appeal mainly to sensory aesthetics, the casual observer will assume that the item is "art". But most notably with Dada, and most extensively with ancient cultural artifacts, and most recently with conceptual art, a casual observer has been faced with what makes something art beyond the limitation of their prior existing appreciation. The "contested" aspect that you mention is not a contest among artists nor among observers who are more educated about art. What does go contested is the notions of "value" regarding different art forms and works -- and "value" is also highly predicated on culture (especially microculture). This is why art appreciation and art history have been taught for decade after decade: to overcome this limitation.
Casual observers are no more likely to understand the artistic labor in art photography than they are to understand the artistic labor in literature. When they don't, they incorrectly presume that what they don't recognize doesn't exist. The talent of an artist for doing something too difficult for the casual observer leaves vicarious experience of creativity an important factor that the observer can get from "reading" the artwork". But when they don't actually know how it is made the experience can be frustrating or dismissed. When they DO recognize the artistic labor, their experience of the work is hugely elevated.
What they need to understand first of all is that photography is a medium in which art-making occurs as well as other activities. The products of that art-making, as so clearly exemplified (at least for Western audiences) by Duchamp and Warhol generate experiences that are highly context-sensitive.
I've been saying "casual" so let me be specific about that. It isn't just the notion of someone being relatively uneducated about art. Here the key issue is that the distribution of imagery and the opportunity for image-making have both escalated by orders of magnitude thanks to what we can call an economic democratization of both tool access and exposure to visual works. But it's important to avoid conflating this idea of "democratic" with the objective reality, which is that photography's presence is dramatically more public. In that public, we can always ask anyone "what would you like 'art' to do for you as an experience?" This does not contest whether "photography is an art" (a phrase that dissolves the more you think about it); rather, it frames an observer's identification of which items among many meet their experiential desire.
One last point: non-fiction photography is no less capable of being art than is non-fiction literary narrative. To really come to grips with the limitation of Sontag's critique, it's important to dispel the superficial focus on "realism" and deal with non-fiction.
