The Vineyard of the Saker: Just the baseless hypothesis of an uninformed amateur, nothing more: First, a disclaimer: I am not a pilot nor an air traffic controller, and I never served with air defense units. I did spend some time with an airforce, but my role was one of electronic intercept analysis. So what follows are just the musings of an uninformed amateur. Caveat emptor.
What I will try to do here is present a possible scenario which takes into account the basic facts established so far. Here goes:
The plan was for the Ukies to shoot down the MH17 using a totally inappropriate aircraft: the Su-25, which is a "tree hugging" close air support aircraft. Why? After all, the Ukies do have some Mig-29s and even some Su-27, but these are much fewer in number and much easier to track. A few of them have been seen in the skies over Novorussia, but only rarely. In contrast, the Su-25 has been a ever-present presence in the skies ever since the conflict began. Some say that the Su-25 cannot fly over 7'000m. That is not true. The problem is that it's cockpit is not pressurized, but the airframe itself has powerful engines which can bring the aircraft well over 10'000m. All the pilot would need is some good warm clothing and an oxygen mask. No, the big drawback of sending the Su-25 so high is that it's engines are not designed to be used at that altitude and that it will not be able to fly fast enough to catch-up with a cruising Boeing. But what if the Su-25 carried a missile?