In science -- including forensic science -- nothing is factual because nothing in science can be proven. (Though things CAN be disproven). Proofs are only valid mathematics.
However, statistics can give the probability of something in science being factual.
So EVERY decision humans make as to whether something is factual or not is based on probability of it being a fact... not on it being an actual fact.
Yet most people -- even scientists -- claim it's a fact that the sun will shine again on our side of the world tomorrow. Even though scientists know that, technically speaking, the odds of the sun shining tomorrow are less than 100%.
Similarly, things that happened in the past cannot be proven. The best a human can do is know the odds that something happened.
Yes you can. That is common practice... for good reason. For example, we always ignore the few people who claim the sun won't shine tomorrow. Even though they might be right.
I'd say that anything with probability of greater than around 1% is "plausible." Certainly not likely, but plausible.
Whether or not something is considered true or not depends upon the odds of it being true. If the odds of something being true are 99.9999%, most people would considered it to be true.