> modern aircraft stay airborne for a long time even without any active controls because of fly-by-wire
Perhaps, but they don't stay airborne at all when the front of the aircraft is missing. And since the controls are on or around the flight deck, losing that would take the FBW offline anyway.
> The goal was apparently to silence the crew and prevent calls for help
Pffft. Nothing silences a crew like blowing them up with a missile. Just ask KAL 007.
> They note that BUK missile makes a very brightly visible plume
No it doesn't. The booster is very smoky but the upper stage is pretty clean firing. Here's what a missile actually looks like:
https://commons.wikimedia.org/wiki/File:F-105_dodging_SA-2_over_Vietnam.jpg
Note that even though the engine is still firing in this case, there is no visible trail. That's not always the case, but just like any aircraft, the trail is caused mostly by physical effects on the atmosphere and thus highly dependant on the state of the weather.
> does sound odd that no one got any footage of the missile
Oh come on. Next time you're walking around, see how many people are looking up at planes. And how many of them are taking pictures?
> Overall, the case is getting stranger with every relevation
Sure, if you know nothing about aircraft, missiles, photography and are prone to believing conspiracy theories.