Interflug.
It does not appear to have occurred to you that you may not be fully informed. You are building all your logical house of cards on the fallacy that you know all the information there is to know. That is demonstrably false; all you know is what has been publicly released and as anyone who has worked with defense radars and governments will know the publicly released information is the minimum possible. Furthermore, if the Malaysians feel that a crime may have been committed there will be even more reason to keep information that is known secret.
The RAT-31 radar as a stack beam radar will have been capable of discriminating the altitude of the aircraft it may even have had it tagged as MH370 and tracked as I suspect that it may track every single aircraft in radar range. There may also be cooperation from Thai military who would not want to admit that they have cooperated.
So based on your false assumption that you have all the information, do you really think that your claim that there is no real information after the aircraft stopped cooperative surveillance is 'logically supportable'?.