"not sure how it marries with all the altitude changes in the plane's flight. Were those altitude changes (flying very high and then very low) speculation as to how it could get through radar, or were they actual stats from the satellite?"
Well my theory is that they flew high to induce hypoxia among the passengers, so they were out for the count & couldn't resist. However, DH who's an aeronautical engineer, said this wouldn't be an issue as the cabin is pressurised at a constant level. Anyway, as per the Helios flight 2005, there was an airconditioning/cabin pressure button that wasn't selected properly in the pre-flight checks, so maybe cabin pressure is controlled from the cockpit (those at the controls wearing O2 masks).
I reckon they flew low (29,000) so theycould watch for planes going past NW-wards, both out the window an on their screen, then hop in behind the SIA68.
"How would anyone on MH370 have known that SIA68 took off on time at might?"
Well, once they're up in the air they have an anti-collision screen with the surrounding aircraft with their callsigns/flight no's... I'm sure they'd have the collusion of ppl on the ground to contact them... maybe the hijackers had a private satellite phone with them?
"Does this article describe a similar scenario as KL?"
No, the Telegraph article decribes flying so close to the ground like military jets do, that they're below the horizon as far as military radar is concerned and can't map them.
Hope that explains things... hoping Goldie will be back later and explain much better!