If a Marciano fight was suggested to Robinson, when would it have been?

But I've covered that in my original post. If this had been considered it would have been (obviously) either before or after the Maxim fight so I'm going to address both.

Before the Maxim fight: A move to the heavyweight division may have been considered before there were any plans for a fight with Maxim. This would mean Robinson would skip the Light Heavyweight division to fight Marciano who wasn't even the champion then.
Why would he go up to the Heavyweight division for a non-title fight?
If he did skip the Light Heavyweight division (which is highly unlikely) he would surely only do so after regaining the Middleweight title from Turpin and that would be to fight the Heavyweight title holder who was Walcott at that time.

He may have considered potentially challenging for the Heavyweight title if he beat Maxim but at the time that would still mean fighting the winner of the Walcott vs Charles fight 3 weeks earlier, not Marciano.
Robinson mentioned in his autobiography that his trainer suggested this.

After the Maxim fight: There is NO WAY he would be thinking about fighting a heavyweight after his very unlucky, unsuccessful challenge for the Light Heavyweight title. He nearly died of heat exhaustion and instead retired and didn't fight again for over two and a half years.

So there is no scenario where he fights Marciano in 1951 or 1952.

The only other possibility for a fight with Marciano is after Robinson made his comeback but he would need to at least regain the Middleweight title first. By the time he did this Marciano had already had his last fight and Robinson was required to defend the title against Bobo Olson anyway in a rematch.
Marciano announced his retirement a month before Robinson successfully defended the title against Olson.

Again there is no scenario where Robinson fights Marciano, there is no mention of it even as a potential bout in his autobiography.

I don't think the possibility of a Robinson vs Marciano fight was even discussed.

It's a myth.