BAckdating ESA in

One of our new tenants has learning difficulties.  He lived with his mother until she died in 2000.  Her will left the property to her other children but giving him the right to live there until he died or was unable to. 

Fast forward to 2022 - his health deteriorates and he moves into our Extra Care facility.  We find he is on ESA and PIP, but not in receipt of the SDP.  p[resumably he would have been on DLA previously. 

Having written to ESA to ask for the SDP to be backdated, I was informed that he has an Appointee.

So,  having read https://assets.publishing.service.gov.uk/media/5ae2d53ced915d42f7c6bac2/CE_3532_2017-00.pdf in a previous posting,
a)I’m not clear how far back the SDP can be awarded and
b) whether having an Appointee whose duty it is to manage his benefits, would affect the decision in any way?