how do we know that what they are saying about the radar track/waypoints/alt is true?
No, we don't know 100% but for now it is considered a "fact", other agencies (NTSB, AAIB, etc) do not question this yet, Malaysian seem sure they got it right. So if you want to come up with a theory better use what today is considered a "fact" and do not bend facts to your theory... Malaysians did contradict themselves but in rather minor details.
Not one other govt (Thai, Vietnam, Indonesia, Singapore) have confirmed this radar info.
it would rather be impossible for those other govt to confirm what Malaysian radar saw, not to mention their radars were probably much farther from the target. And who knows if Thai radar capabilities are even half as good as Malaysian radars.