I am sorry you are going through this. My husband is currently detained under a Section 3.
Patients are indeed entitled to leave under Section 17. This would have been assessed by her responsible clinician (psychiatrist) and it is also an important part of recovery and seeing how people do in the community. It can be withdrawn at any time if the risks change. The nurses on duty will also have assessed your mum's presentation before allowing her to leave the ward. I admit, that it does seem very quick, and usually they start with escorted ground leave, but it is going to be different for everyone, of course.
It is very common for people in psychosis to disengage and refuse loved ones to be given information. Often due to the belief that loved ones got them 'sectioned', or haven't done enough to keep them out. It happens all the time, and can often be due to Anosognosia. I know how brutal it is when the person you are trying to help is making it difficult to let you, and when they become uncharacteristically 'mean'. I don't think I will ever be the same again after all of this.
There should be a weekly MDT meeting, ask the ward nurses if you can be invited to these- your mum will need to consent. As for discharge, it's best practice that a family member is involved in this process, and her nearest relative should be consulted when this happens at the very least.
The nurses should have been able to give you some information, I would make sure that a data protection form has been completed if you mum has now consented, as the nurses will not say much without that form being on the system.
It is a battle, getting people to communicate and listen to you. Mind has a phone line that you can call to discuss any legal matters around being detained that you might find helpful.
I really hope your mum improves soon. Please feel free to PM me if you want someone to lean on.