>Could it be that the mound was mostly finished before the stone structure was fully under way ?

Mebbes. It would make corbelling with heavy stones easier I suspect. I wouldn't want to have had the of dropping that monster passage slab down into a hole in a mound though.

It seems totally plausible that monumental mounds could have been built on top of pre-existing non-moundy monuments. One could even be tempted to say it would be likely for a spot of prior significance to be deemed a suitable place to go through all the effort of piling up vast quantities of soil/rubble whathaveyou. Look at that place with all the trilithons, if stones put up in places that were already special, why not the same with soil (for those who were of a mind to pile soil instead of lugging rocks)?