Julian, you think I liked the Byrne rendition?! I had suspected you were off your rocker when you were making anagrams of authors' names, but now I am sure... SURE!!!!! <laughs maniacally>
Aw, relax, I'm just busting your chops.
Actually, it's more frustration turned to muderous rage that I can't find an anagram for UNCLE MXY (no seriously, try it).
So, if the comics strongly implied that Smallville was East Coast and very near Metropolis and did so for DECADES...well, that's it, then.
The DECADES is fine for emphasis of your point, however, the only pre Bronze Age reference you cited in your construction of a misconception was the original Legion story that stated Smallville was close to Metropolis, not where either was...so if Maggin was the first to state the east coast in his seaboard travelogue, we are talking about 10 years before Crisis here...
That leaves about 30 some years where these locations were unspecified, probably deliberately.
You keep on saying that, but there have been many occasions in the Silver Age (I can't speak with authority on the Golden Age) where Metropolis has been shown to be coastal: Metropolis is shown to have harbors and to be occasionally menaced by a tidal wave. Some of the early Superman/Aquaman team-ups take this for granted, for instance. There was one early Supergirl story where Kara was trapped underwater by Red Kryptonite, and was able to battle crime from the sea.
Metropolis on several occasions, is also shown to have sharp winters that get snow, and hot summers; if it was, for instance, a city in the Southwest, this would not be AS true, and the Summers in places like the the Dakotas and Canadian border are not as pronounced, ruling them out, too.
And cities in the South, even sophisticated, cosmopolitan cties like Atlanta, have a fundamentally different character that would be noticed, and wasn't present in Metrpolis.
Even discouting the Bronze Age details like Metropolis having landmarks that correlate to New York (WGBS building instead of Rockefeller Center) would the location of the Northeast not be a shrewd guess?