I know he might be gone, but what does this mean?
My WAG:
A person’s head is/was sometimes called a coconut. If someone is ‘out of his head’, then he’s ‘off his nut’, or ‘nuts’.
I agree with the idea that ‘bananas’ comes from the ‘insane’ behavior of the stereotypical chimpanzee.
As for ‘fruitcake’, I think we go from ‘coconut’ (head) to ‘nut’ (head) to ‘nuts’ (‘out of one’s head’ => ‘crazy’), and come to ‘nutty as a fruitcake’. So I think ‘fruitcake’ comes from ‘nuts’.
Again, these are unsubstantiated guesses.
The “great banana hoax” was 1967. I doubt that’s the connection - I’d be more likely to go with the “acting like a monkey” angle, although somebody quoted a Miriam Webster date of 1968 upstream. I’m not sure of either one - bananas have turned up in various humorous contexts for ages simply because the word SOUNDS funny (and the fruit looks funny and rather phallic). The “Yes, we have no bananas” song was around in the 1920s. It was composed as a novelty song during a banana shortage, but had there been a shortage of, say, apples, the song would have been less likely or would probably not have taken off.
The lights are on, but no one’s home.
I think that’s the best explanation. We used words that really off the wall to sound funny.
I could say that she lost her Bingo card and you’ll probably crack up from it.
My guess would be that the board software used to display a red envelope for a thread with unread replies. So his post was the equivalent of a ‘bump’.
But some old-timer would have to confirm that.
17 year old thread? Some of you were just babies then.

The “great banana hoax” was 1967. I doubt that’s the connection - I’d be more likely to go with the “acting like a monkey” angle, although somebody quoted a Miriam Webster date of 1968 upstream.
I was skeptical of the connection at first, but then I tried the Google Ngram viewer and both “go bananas” and “he went bananas” scarcely appear before the late 1960s. Woody Allen’s movie Bananas is from 1971.
I had a look at my favourite etymological websiteand found the following:
What of to go bananas? It burst upon the world in the 1960s and became a fashionable, not to say faddish, term in the 1970s. Its heyday is over, perhaps thankfully so. But nobody seems to have any very clear idea where it came from. Was the idea of something bent at the root of it, so that a person was being driven mentally out of shape? Or was there a mental image of an over-excited ape clamouring for his daily feast? Or was it a more subtle image connected with the older phrase to go ape or even to go nuts? You can go crazy thinking about this stuff.
He also remembers, as do I, a derisive use of bananas in the 1970s for corrupt London policemen, on the grounds that they were yellow, bent, and hung around in bunches.
Holiday Lament (The Fruitcake Song)
Nobody likes a fruitcake!
Nobody likes a fruitcake,
We all need therapy.
It drives a cake bananas,
When a person hates her guts.
No wonder all our name has come to mean
Someone who’s nuts
The red envelope was where someone stored the answer to “14 k …”.

“Crackers” comes from “crack brained” (eccentric, slightly mad) nothing to do with biscuits.
In Victorian times in England, the “-ers” formulation became a common diminutive/humorisation of a root word - so “stark naked” became “starkers” (or “harry starkers”). It’s still sometimes used among groups of male friends to make a nickname out of someone’s regular name.
The one association of food to madness that I can think of is one that doesn’t feature in modern slang - mouldy bread and ergotism.
Behavioral problems are associated with traumatic brain injuries; so, perhaps, ‘a crack on the head’ might lead to being crackers.
Bonkers, too, now that I think about it.