Grandparents first of all have to get the court's permission to apply for contact (as I think you know). So at this stage you are dealing with the permission only, not contact itself.
I recently acted for paternal grandparents who got leave where DF was not seeing the child at all but where GPs could show a high degree of commitment and good relationship with the child. DM had a very difficult relationship with DF and felt unable to deal with the GPs.
The starting presumption will be that it is in a child's interests to know and have a relationship with his/her extended family. So you will have to jump that hurdle at the leave hearing.
Any presumption is of course rebuttable.
The first step in proceedings is a safeguarding report. A CAFCASS Officer will call you (and GM) to ask if you have any concerns. This is where you tell them that your DS is known to social services and that the SW thinks that contact would be against DS's best interests. The test the court applies is always the child's welfare. CAFCASS will then call your SW to get more info. They then do a brief safeguarding report which is seen by the court at the leave hearing.
So if the SW thinks that it would be detrimental then they will be able to tell CAFCASS that.
Not asking for more info but I couldn't say how the therapist evidence would be relevant without knowing more. But your solicitor will be able to advise about that. Primary carer's mental health will always be top priority so if this is going to really affect your MH you will need to demonstrate that.
This may never get as far as a substantive contact application because grandmother may fail to get leave.
Re the narcissism, just be aware (as you probably are) that is a very difficult point to get across and to prove. I know because I am currently litigating against a narcissist in a personal capacity (in fact I think he's a psychopath, the distinction is very fine).
Good luck.