If I might re-iterate my comment made on another thread - as soon as one introduces logical thinking into the process, the traditional ideas fall apart. Not only that, but the current views of Bauval, Hancock and West in their agreement with Hawass regarding the Khufu construction, are open to serious criticism.
This does not mean that alternative answers are readily available - in fact, quite the opposite exists - the logical process takes us into 'don't know'.
If I could comment on your association with the Old Kingdom, it is also quite logical for the Old Kingdom to have inherited ideas from an earlier period. If Giza is earlier, then concepts associated with Giza would also have been inherited into the Old Kingdom.
It is factually correct that scholars advanced in the study of Egyptology are not necessarily in agreement on the idea that the Re/Horus concept was original to the Old Kingdom. If you hunt around, you will find scholars who are of the view that the Re/Horus concept had been inherited by the Old Kingdom. Therefore, the glyphs relevant to these concepts would also have been inherited by the Old Kingdom.