Is it permissible to volunteer to testify in order to deny a charge of premeditated murder against people whose innocence one is certain of, especially since the accusers have admitted the invalidity of the accusation?

The testimony must be rendered with certainty about what was witnessed; it is not valid based on doubt or mere conjecture. The witness's reliance is on certain knowledge, as Allah Almighty says: (Save him who testifies to the truth and he knows) and (And we did not testify except to what we knew). Whoever testifies to what he is not certain of has committed false testimony, which is one of the major sins that incurs Allah's wrath and displeasure. The Messenger of Allah (peace be upon him) equated it with associating partners with Allah. It is forbidden to resort to false testimony, even if the intention is to establish a right. Therefore, witnesses are not permitted to testify to what they did not see, but they are permitted to testify to what they knew and heard.