You are not a HV by any chance are you?
They have to consider the whole picture and take into account your circumstance,your previous work record etc,
The fact your line manager knew you were struggling with personal issues..did he/she do anything offer to refer you to occ health? Suggest anything else? If not this needs highlighted your employer has a duty of care to you and issues in your personal life can impact on your work.
The fact you went off after your appraisal means your manager should have put a plan in place when you returned to work following your sick time since they were obviously then aware of how bad you were feeling.
It is good that you booked some counselling.
If it comes to a disciplinary hearing would your colleagues speak up about the workload to support you?
Even though the union might not act on your behalf due to it being a on going investigation when you joined if you joined you could access free advice/ telephone helpline etc and sometimes helpful reps will speak to you and give you some pointers even if they can't directly support you.
It's also good you are not being taken away from client contact.
Try to have some quality time with your children this weekend and a glass of wine tonight when they are in bed if it helps.
I really don't think it will go to a hearing there is also a time frame for investigations as well so it can't drag on forever.- 'This too will pass'