John- Good thoughts all, but I'd add the following.
(a) Acute disease in not symmetrical.- Often true, but not necessarily so. It may present symmetrically if the target tissues are themselves symmetrically located.
(b) Symmetry indicates systemic trauma or failure of a specific subsystem.- Sometimes true, but not always, as indicated above
(c) If this holds true, we would be looking at a secondary fungal infection, with either a systemic trauma or failure in an organ that contributes to the health of the gills right?- I agree. Almost all fungal infections are secondary, in the sense that they arise opportunistically in tissues that have been compromised in some way, either directly or indirectly. But it's complicated by the fact that a single stressor (e. g. immunodeficiency) may present in many different ways.
(d) It is commonly thought that fish can only absorb through osmosis 100-200g/mol or smaller...- Osmosis is not the only mechanism by which antibiotics enter the system. If this were true, none of the antibiotics with molecular masses greater than the range you describe would be effective, yet empirical observations indicate that many high m. w. compounds are highly effective.
(e) I would believe it likely that nitrofurazone would be a better option if treating an underlaying condition of systemic failure, and not a dermal layer acute problem...- I'm not following you here. Antibiotics target adventitious infections that occur when tissues are compromised in some way; they do not affect the root cause of the problem. Once the infection is under control, one looks to identify and remedy the underlying cause, which in aquarium fishes is often called 'stress,' an umbrella term that encompasses a multitude of causes, including poor water management, overcrowding, inappropriate social and/or physical environment, etc., as you know.
(f) It's possible though since this guy is a sand sifter; he pulled something gnarly through the gills and did get some symmetrical acute trauma...- Just so, see (a), above.
(g) ...a good reason for recommending the erythromycin...- Nothing complicated here, just empirical observations, i. e. it generally works as well as or better than other medications. In practice, you can take some of the guess work out of this by using commercial preparations composed of mixtures of different antimicrobials. This 'throw-everything-at-it-but-the-kitchen-sink' approach can be very effective.