In legal speak, if things aren’t word for word, fact by fact, then you can’t say for sure, correct? Any slight assumption being made means it’s in ambiguity.
I'm not going to comment on the legal point of view (I'm not in the field), but from a scientific point of view (I fit in here; scientific = hard science), there's no problem in considering assumptions.
A good scientific practice will act by choosing the assumptions that seem most plausible based on the evidence we have at the moment. If, in the future, new evidence indicates that the assumption made was wrong, we go back and start again. This is the daily life of those who work in science (scientists, professors, journal reviewers, etc.).
In the case of DT, we don't know many details. But we can think about premises/hypotheses and consider whether they are acceptable or not in light of the facts we have and some assumptions.
For example: the change of drummer occurred by divine imposition as in the old testament. It is a hypothesis that explains the observed fact (fact = MM outside the DT; MP back in the DT). Does it seem like a good hypothesis? I don't think so, as it doesn't seem possible to confirm or falsify it.
Another hypothesis: MM, of his own free will, decided to leave DT and with the drummer position empty, the guys in the band chose to bring MP back.
Can this hypothesis explain the fact? Yes it can.
Is it capable of being falsified? Yes it is.
With the evidence we have, can we say that it is 100% false? We can not.
Can we say that it is 100% true? It makes no sense to try to show that a premise is true, it only makes sense to try to prove it false (I'm following Karl Popper here).
Does it seem false, with the data we have? Yes it seems. If that were the case, the most plausible thing is that this would be said straight away, as it would be MM's decision and would be easy for everyone to understand (just like when KM decided he no longer wanted to be in DT, we may not understand his reasons , but it's easy to understand that he decided to leave...). Of course I'm making an assumption here to rule out the hypothesis. And there's no problem with that.
Therefore, I would not be afraid to discard this hypothesis in favor of another or others. On this point I differ with the lawyers. It may be that in the future, MM will come forward and say "ah, it was my decision to leave DT, but I didn't want to make it public at the time".
OK, my choice to discard the hypothesis was mistaken, based on an assumption that did not apply to the case. But I didn't have that information previously.
Our social world is made this way. We make choices and have convictions that are based on assumptions that we believe in, some more rational, others not so much.
If people operated predominantly with the idea that "oh, I'm not sure if it is or isn't, so I'm going to cautiously stay in doubt" the number of agnostics would be much higher.