The first hearing will consider all of the issues that have been raised in the Safeguarding Letter, and anything else brought up in court
The court can only make orders at the first hearing when things are agreed, or are in the nest interests of the child. So it is difficult for them to resolve issues which may need more information or which are complex.
It is common for CAFCASS to be ordered to undertake a Section 7 Report, obtain drug or alcohol tests and typically people argue that their ex has mental health issues, because they are clearly mad, requiring confirmation from their GP that they are not.
The first hearing sets the scene for your case. The better prepared you are for all eventualities the better and the less likely there will be any delay in resolving your case.