"At least the radar echo, if not faked, is showing another aircraft. Can be a MIG-29 or can even be anything else."
Radar echo is just what it is, an echo. Echo's could come from clutter, reflections, even birds. Being an echo is no proof of being an aircraft. In this case the most likely explanation is that the echo's are parts of MH-17 when it fell apart.
If you have doubts, please read below.
Russian ATC lesson 101 ? the phantom SU25 | who is strelkov - the many mysteries of the Russian Involvement in Ukraine