Virtual terminal location?
First, kudos to RichardC10 for an impressive job and no doubt a lot of hard work.
One item that I find odd is the 718 km altitude of the virtual terminal (VT). This seems a very unlikely choice for humans to make - an altitude of 0 would be far more likely. I suppose that they use a VT plus another unstated constant time correction, and that appears as the VT in RichardC10's analysis, but it is odd.
Is it possible that the VT is zero and some other factor is hidden in there to make things work out as if it were at 718km?
It's most unfortunate that Inmarsat didn't publish the full analysis.