zoggs when you ask about stating the boss had already decided the outcome do you mean what you said about the boss saying to you that the outcome will be devastating for someone? Do you know who that someone is and is it that person who wants you to make a statement?
If you have witnessed bullying a statement to that effect would normally come as part of a grievance hearing for a grievance raised by the person being bullied. Is anyone considering raising a grievance about bullying or anything else?
I'd be inclined to keep out of it if you can, unless there is something concrete that will make a difference for someone. I think the worst thing about what your boss said to you was the fact that it was completely inappropriate and unprofessional to have that conversation with you. In terms of having already decided the outcome, if it is a black and white evidence case with no detailed lengthy subjective investigation, knowing what the outcome is likely to be isn't so bad. If that outcome is unreasonable your colleague can appeal it as such.
In other words, the boss did do wrong discussing it with you, and if his/her mind was completely made up inappropriately, without having heard all the evidence, and if you were positive that regardless of any evidence brought by the employee he/she would not have changed their mind, then fair enough, and you might want to consider making a statement. But only do so if you feel what you have to say is concrete evidence of something and makes a difference, and having considered the implications for yourself.
But it sound sounds as though there is plenty for you to worry about in terms of whether you'll be 'next', and involving yourself too much in 'defending' colleagues possibly won't help you.
If the sanctions taken against your colleagues are too harsh, or a policy is not followed, your colleagues can appeal the outcome and then take further action if necessary/appropriate.
Sorry you are in this situation - even when it isn't you it's tough, and very draining. If you don't feel up to work you should consider whether to get yourself signed off sick, with stress or whatever. I'm not going to encourage you to do so if you are not genuinely ill, but if they might be considering disciplining you and you are sure there is nothing they should be disciplining you for, then a short period off sick might put them off going after you for a bit, as they might feel vulnerable targeting someone who is signed off sick. Something to think about, but read your sickness absence policy carefully first, don't do it for too long, and be sure that if they are paying occupational sick pay, they probably have the right to get a second opinion if they feel it's necessary.