The difference between the unlocked square and diamond is that one predicts the location of the aircraft (which can not be found due to the lost transponder), and the other is the reflection of the no longer transponding aircraft falling to the ground (an active signal return). It only ever shows one aircraft and a prediction, never two.
If only that was so easy. Which one, predicted or actual plane, suddenly flew orthogonally to the original course, as seen in Russian radar video, while still being tagged as MAC17?
It is possible that radar kept tracking torn off pieces of the roof, which fell separately and quite far away from the rest of the plane, but that is just ONE possible explanation, not a known fact.
It needs to be tied up to Dutch investigation data - when, where and at what altitude exactly the roof was separated, and whether possible pattern of it falling down matches with what is seen in the video. Was the piece big enough to be caught by radar? Why did it replace the main body of the aircraft?
There's a lot more to either of these scenarios that can make them ultimately implausible.
There could also be information about military planes in the area but Ukraine managed to get veto power over what is released to the public and what is not, don't forget that aspect of investigation, too.