I haven't seen it disputed that the plane turned in a direction which was taking it towards Diego Garcia. As you say, it turned west. The official account is that it then made a further change of direction.
As for the eye witness accounts, what I haven't seen is any account of which plane they saw, if not this one. Unless you think they're just making it up, of course, and didn't see any plane at all.
Regarding malfunctions, we are told that the change in direction was deliberately programmed in, and was not done in response to an emergency. Do you think that is untrue? Yes, malfunctions happen. But we are presented with an account which means that this change of direction was deliberate, which does rather undermine the case that a series of malfunctions was the issue here.
Don't you find it a little hard to believe that the country with the best capability for observing what is happening has no information on strange and unusual movements of a plane in a sensitive area? Do you think that although they can apparently gather vast amounts of all sorts of other data, they can't spot a plane flying in an unapproved direction in an area where they have a spying base? Does it really sound in the slightest bit credible?
I would suggest it's pretty obvious the US has information which they aren't sharing. Which is why people are exploring ideas about what that might be, and wh
It is certainly possible that the plane was picked up by the base at DG, and if it failed to response to requests for contact for whatever reason, I would imagine eventually the USAF could have taken the decision to shoot it down. I wouldn't imagine that terrorists generally jump on the radio and say "yes we've captured a plane full of Chinese nationals and are going to land it on top of your control tower you yankee pigs!"
If the above theory is true, you can certainly understand the US silence, because they are no way going to own up to shooting down a plane full of Chinese intentionally or otherwise.