The posters hear had to gently beat into my brain when I joined this insightful group a year plus "get your mind around their belief system first before understanding/explaining why they did things. It was one of the most valuable early lessons I recieved here.
1) There is plenty of documented evidence they saw in the Scarab beetle elements of eternal life. That is probably plenty of reason for them to meld it into their myth ladder.
2) While they had full knowledge of the human skull (look at their medical trepanning) there is no evidence they respected it as an achievement. If eternal preservation of the corpse was the penultimate goal, wouldn't a naked skull be viewed as failure to achieve the goal? Such a poor job of preservation that the skull was exposed after mortifying would seem to the AE to be a very bad thing.
Why then, would they choose such a good example of failure to represent their afterlife beliefs?
Jammer