My problem is ~ I don't know how accurate that Red Arc map is, and I don't know whether to trust it or not.
Unfortunately that arc could be even longer, since they are assuming maximum fuel consumption (max speed). And there are some further complicating factors -- the reports of wild gyrations in altitude, which would have also dramatically affected fuel consumption.
If it were me trying to get somewhere, and I had confidence that I could defeat the various primary radars in play, I wouldn't go at mach .89. I'd stretch the fuel consumption to increase my options; for contingency planning. That is, as long as I could still get to my destination almost entirely under darkness. I think the max-endurance range of mach numbers would be about mach .68 to mach .73, depending on the weight (long range cruise slows as fuel is burned off).
It would have been easy for this airplane to cloak itself visually. It was during the night, and they would have turned off their strobes and navigation lights with a couple of switch pushes on the external lighting panel. No other airplane would have likely seen them. They probably had a contrail, but nobody would have seen that either.
Question for you. How close to each other would two aircraft have to be to appear on the radar scope as one target? I suppose as you implied with the AN225 anecdote, it depends on the distance from the radar. How far away from the radar site would two 777's flying close trail have to be to appear as one target? I'm thinking of 200 feet spacing, both vertical and longitudinal. That would be a safe distance that even someone with no formation training could maintain, and even if they overshot speed-wise or bobbled with altitude that would be enough separation to avoid a collision. Would a typical controller notice a double image under those conditions right away?
I think the more recent news reports are making this scenario we are examining more likely, by the day.