The red line passing through G3 is not parallel to the other lines (as agreed)
Actually; more accurately, none of each of the three lines is parallel to any each of the other ones.
but there is absolutely NOTHING that dictates that from this centre (i.e. the Taylor Point), a cord could not have been stretched in this manner, producing the eventual 'gap' that results in the differential between G1 and G2 base sizes.
That's not quite correct, Scott. Perhaps you may not be familiar with this, but drawing lines on a small diagram like this is very different from the alignments that are actually existent in the structures themselves. What looks good on 'paper' may not actually work in reality. If we take a look at the angular expansion rate produced by the three unparallel lines we find that the base area of G1 is actually about 36,471 square feet too big for the Giza-3 to conform to your proposal.
As you can see your alignment theory is more off than your image suggests, even if we somehow accept that they misaligned the 45* method by 55 feet over the course of the G3-G1 triple axis.