Okay, as mentioned, Vol. 2 of the Martian Manhunter Showcase is in some ways even crazier than the first one; but why?
It's clear that the editorial/authorial powers realized they'd painted themselves into an uncomfortable corner. They'd created a nearly infallible character whose powers were a mash-up of Amazo, the Composite Superman, and the Phantom Stranger. His only weakness was the most absurd one any character has ever had. Yellow? Wood? Those at least had some sort of ad hoc logic to them. But the Martian Manhunter's vulnerability to fire defied not just physics, but sociology and common sense. Fire has no radiation-like "emanations"; at a distance, there's no difference between fire and any other heat source. The Martians built a high-tech society without... fire? Maybe they used "Martian vision" to melt metals or just finger-snapped machines out of thin air, like they do ice cream cones?
PLUS, this super-scientific society has no way of rescuing one of its people, even when it's established early on that they know he's on earth? No, it's obvious that the Martian Manhunter was a rather hastily cobbled together character, and a difficult one to write. Despite J'onn's godlike powers he spent most of his time fighting crooks in hats, and laughable losers like the Human Flame, Mr. Moth, and the Human Squirrel. His only substantial challenges would come from other planets like the Venusians or the Martian Mandrills (yes, really). Something had to be done.
The recognition that J'onn's powers were out of literary control led to one early limitation: exposed to a chemical by a Martian bad guy (B'rett, as I recall), the Manhunter could no longer use his powers while he was invisible. Not only did this prevent the obvious question ("Why doesn't he always remain invisible as matter of course during combat?"), it forced him to 'come out' and become a public hero.
In Vol. 2, the Martian Manhunter even started to fly as a pure and simple power, whereas before he always had to suck or blow himself into the sky. And now he even had a superpowered sidekick (more on that subject later) to help him.. Something beyond the 'invisibility limit' was required.
The writers (well... writer, really) came up with an idea: J'onn's 'Martian powers' worked only when he was in his "Martian form" (although this was in direct and stark conflict with every previous Martian Manhunter story). Now, when in any other form, he had only the powers of that other form (and, of course, his power to shape-shift!). This helped a bit at first... JJ kept having to disguise himself as humans, and in so doing became as vulnerable as they are.
Unfortunately, the concept kind of backfired; when face with a superior adversary, the Martian Manhunter could simply assume the form of his opponent... and did.
So they killed him off. Not the Martian Manhunter, but John Jones, his civilian ID. He was "killed" when he and a car were eaten by corrosive cloud being. Things like that happen in Apex City.
Rather than concoct an escape story or one of those Clark Kent-style "Superman saved me when you weren't looking, Lois!" yarns, the Martian Manhunter simply said, "Huh, oh well. So much for my carefully constructed private life, my friends, and my job. What's for lunch?"
But this coincided with a move for MM. He no longer needed his job as a detective because he was no longer in Detective Comics. Now he'd be the back-up in House of Mystery (which was, at that time, a sci-fi title, not a horror title), so his adventures became more mysterious and less detective-y. Enter the Idol Head of Diabolu, a magical Pandora's box that spit out a different, crazy magical threat like clockwork once a month....
to be continued.