But presumably not a situation where the attackers mistakenly thought they were shooting down an aircraft that was about to attack them.

They definitely thought it was an Ukrainian military transport plane. I don't see how the distinction between enemy military transport plane and enemy fighter/bomber is of any importance here.

How do you know what "they" thought?

Version about rebels taking passenger plane for AN-26 seems plausible, considering that rebels themselves seemed to believe it in the first minutes after the event (I am judging by the reaction of their mouthpieces on Russian internet which I happened to see with my own eyes).

Anyway that's not the point, the point is that even if (people may still doubt) the rebels downed the plane, at worst they thought of it as of an Ukrainian military plane, ie they had no intention to kill noncombatants, it was definitely not an 'act of terrorism' as Ukrainian govt wants to paint it.

How can you know if "mouthpieces" are real rebel commanders or just some hotheads in their mommies' basements (or Russian equivalents of such)?

I am talking about Boris Rozhin's livejournal ('colonelcassad'). That's about as close as it gets to rebels' mouthpiece, being probably the most popular social media outlet with separatist viewpoint (not to be confused with Russian govt's viewpoint). Of course he is not an official figure of any kind, but if anyone can be trusted to be able to tell legit sources from 'hotheads in their mommies' basements', that's him.

It's a journalist from Crimea, Nobody you should consider to be fully informed with what rebels do. I can imagine he's now a kind of a war reporter, but in wars there are a lot of rumors. Nothing can be more probable than that somebody actually saw the crash and produced the rumor that that's an achievement of the rebels and the journalist repeated it.

Oh, it's simple. Separatists were posting how happy they were about bringing down the Ukrainian military plane. In some hours it turned out it was MH17, the messages were deleted then. (I've seen the post myself, not a screenshot).

How can you know if separatist on the internet are real rebel commanders or just some hotheads in their mommies' basements or (gasp) other intelligence agencies? How do you know that nobody took over their digital identities? We're on HN we all know how easy it is.

I assume I know that since that account has a rich history of timely reports, quite a few with unique media content. The account is being continued to be filled with the same inside information which indicates it is unlikely to be hacked.