The most important thing bringing answers while we are travelling across the globe is always logic. Logic in its mere intrinsic meaning. Not biased logic we "learned" to have at school or through the media.
that the buried backs are all carved! Do you really think the builders would have carved the backs if the were initially meant to end up buried underground?
We see only one logical answer to this.
Statues were all standing fully above ground so people could see the carvings. Time did its job, and after (tens of) thousands of years, only the heads remained visible.
During our last expedition to Egypt, we noted a clear recurring pattern: the older the constructions, the bigger and the better they were done; without any additional message on them. Sheer flawless stonework, nothing more. It was only later that the next civilization/group of people found those sites and started doing what we refer to as "hieroglyphs". And again later, "we" found all that out.
Is it so crazy to think that the same pattern applies to Rapa Nui? A much older civilization possessed advanced technology to erect those Moai, a later civilization found them standing above ground and carved them, only to be covered by evolving grounds during thousands of years, before "we" found all that out?
until it "became" round? Yet again, thinking we know everything is peculiar to mankind isn't it?