How do DWP / ATOS decide to request medical info?

Questions:
1) On what basis do the DWP and ATOS decide whether they will or won't request medical evidence?
2) Can they be requested to explain on what basis they decided NOT to request medical evidence in the case of my niece?

I am helping my niece by marriage to claim PIP. She is 30 and has been on low rate care DLA since she was 16 for anxiety + panic attacks (very difficult circumstances as a child, also was given Special Needs support at school). She was diagnosed with Cervical Myelopathy in 2015, had an op in early 2016, which helped at first, then further deterioration has set in. Her GP told her she should apply for an upgrade on her DLA, and as she had no one else to help, I stepped in, although I have no knowledge of benefits (but am learning fast). I was advised she would have to apply for PIP as DLA is being phased out.

The call to the DWP to register for PIP was done when I was with my niece, and the speakerphone was on. I was doing the talking for about 50% of the time and could hear everything the rest. There was no mention made of submitting medical evidence. I also took her to two agencies for advice and help with completing the form, and neither agency mentioned about submitting medical evidence. We went to the F2F Assessment and later got a letter stating zero points, with the DLA cancelled immediately.

We went back to the agency who submitted the form and they did what was needed (or rather, what I thought was needed) for a Reconsideration. I requested the ATOS report from the DWP and saw when I got it that there was no medical evidence presented, only the evidence that my niece gave herself. I was furious, I thought that the DWP would request the medical evidence, as they ask for the GP and Consultant contact details in the claim form. I then found out from elsewhere, to my frustration, that we could have submitted medical evidence ourselves, with the original claim and with the Reconsideration.

My niece's GP says that he only gives medical info when the DWP request it, that they have to ask for it first. The wording on the PIP form talks about 'additional info' and at the time, I couldn't imagine what this might mean and glanced over it, but had it said 'medical evidence', I would certainly have noticed that and asked about it. I feel terrible that I didn't know and that this has lost my niece her DLA.

I've now got my niece's medical records and am getting her hospital records, including MRIs, and have asked the DWP for for time to submit this info for the Reconsideration, but I'm trying to get to the bottom of how the decision is made by the DWP and ATOS that medical info will or will not be obtained. I've looked at the B&W Resources, and can see that either the HP or AP can request it, but the DWP guidance doesn't explain on what basis this decision would be made. I want to use the fact that we thought the DWP would get medical evidence as the reason we didn't submit any (or even thought of submitting any) as the basis for presenting evidence now at the Reconsideration stage.

Sorry for the long explanation, I feel so guilty. Can anyone advise?