Everything gets stranger. Much stranger.
You think that's not possible?
How is it possible? In Volume 1, J'onn J'onnz had cars driving across his giant butt, used Martian finger snapping to cure cancer and square the circle, fought the Human Squirrel, and created ice cream cones out of thin air with his mind. Really, where can you go from there?
Well. I guess we'll see. One thing, I'll say; you do NOT go to "Middleton".
I reject the false doctrine of the Middletonites, and I hew to the Apex Creed:
I believe in one city, Apex City, home of the Martian Manhunter, and at all times, whether he's visible or invisible.
I've read both Volumes of Martian Manhunter Showcases, and there is only ONE mentioned of the putative "Middleton" as the home of the Martian Manhunter. And here it is:
And in one J'onn J'onnz, the only-remaining son Mars, created in the Silver Age before all heroes; J'onn of J'onnz, lighted by firelight, very Silver of the Silver Age; written, not edited, being of one substance with the Schwartz, by whom all comics were made.
Okay, in case you don't recognize my megalocranial friend there, that's Prof. Arnold Hugo, a one-off Batman foe (Detective Comics #306, August 1962, "The Wizard of a Thousand Menaces!"), whose biggest accomplishment was failing to create an artificial moon. Hugo's debut against Batman was in the same issue of Tec where Detective John Jones died (more on that later!). Arnold Hugo became the Martian Manhunter *snicker* arch-enemy. Sigh. I guess it was a step up from the Human Squirrel and the Human Flame.
Anyway, it's Arnold Hugo who identifies MM's town as "Middleton", and it's never mentioned again. A hapax legomenon uttered by a notorious lunatic, liar, and loser is nothing to base continuity on! I think Arnold's just raving. Or perhaps Middleton is a suburb or or neighborhood within Apex City.
Regardless... it's still clear we're in Apex City: