An afterlife would be terrifying for me

[Apollo]

these are all offshoots of original ideas of a plentiful happy life

So you're sure that it all started out as a fantasy in a pre-literate society, and then evolved into something somewhat different. 

It's sort of hard to know about the fantasies of pre-literate societies. Other than the fact that this sounds good, how would you go about demonstrating that your explanation is more than a just-so story?

 without giving much thought to what is happiness to begin with and what are its various forms and sources.

The standard Christian view of heaven, as developed by the people I named, gives a great deal of thought to the various forms and sources of happiness. Arguably no one has ever given more thought to that subject than the Greek philosophers whose ideas were taken into Christian theology. 

Have you read the Philebus? or the Nicomachean Ethics? These were influential in Christian theology. The Platonic and Neoplatonic systems which gave rise to the view of Heaven described by Dante, for example, has detailed and elaborate arguments about what human happiness consists of, why we generally lack it, and how it is to be gained. 

So I'm not sure who you're talking about here, but Christians and the philosophers who came before them gave a great deal of thought to this.

Of course this is all guess work based on various prehistoric archaeological finds and how some of the dead were buried or other signs. No one knows for sure but it makes logical sense that prehistoric view of afterlife would be simple rather than complex.

I am not much familiar with afterlife thought works of greeks or Christians but Islamic afterlife is full of rivers of milk and honey and wine and abundant of sex.