Was your Dad kept in hospital under Section 117 of the Mental Health Act, by any chance? Because if he was, then that alone may place the onus on the PCT and LA to fund his care after discharge from hospital. But it would depend on which section he was sectioned under, if you get my meaning?!
And when was the EPA signed? As long as it was before 1 October 2007, then you are required to register it once the Donor loses the capacity to handle his own financial affairs. So you won't have any problems with registering it.
Or is it an LPA, created after 1 October 2007? If it was an LPA, then was it a Property & Finances LPA, or do you also have a Health & Welfare LPA? Again, no problems should exist with registering either of those two LPAs.
But the old-style EPA only covers financial matters - not health and welfare.
If you don't have the Health & Welfare LPA, then you'd need to apply to the Court of Protection to be appointed Deputy - unless you think your Dad still has the mental capacity to agree to and sign a H&W LPA.
As KK said, repeat until you're blue in the face that you challenge the assessment, and insist on all info being given to you. Make sure you're present at it, and do your own assessment too - with any evidence from 'other people'.
Sign nothing; give no details of finances to anyone - and certainly not the SS; don't sign a contract with a care home if they place your father into one; make sure you are involved in the choice of care home, if that comes about. But as KK said, he's best kept in hospital for the time being. If your father is very aggressive, or likely to become so again, then there are very few dementia residential care homes that would be able to cope with him - and he would need a care home with nursing. But be prepared for the fact that there will always be a res. c/h that will accept him, and then decide they can't cope - and he'd need to move again .... and again ... and that can cause him even more disturbance.