The Alzheimer's Society have some very good resources on CHC, which I refer to every time we are reviewed, even though DH is quadriplegic and doesn't have dementia. It's just a good primer.
My advice is to treat it like a legal case. Do your own scoring beforehand so you are prepared to talk through needs in each domain. If you can, make sure you invite at least one of your HCPs. We generally get our GP and key district nurse to come along - they can be invaluable in challenging inaccurate statements (which are not necessarily bad practice or anything, just that having someone really familiar and properly qualified is generally useful.). You can invite anyone you like to the assessment so get people on your side.
Understand that it's not about how sick someone is. You could be dying and not qualify for CHC. It's a particular set of criteria that you must meet, that is all. The key things to consider are complexity and unpredictability of health needs.
Social needs are not worth arguing on as the decision is about whether the care required is primarily a health or social need, and social needs means council funding. (You might like to read the Health Acts, esp 1948, 1970, and 1990 to get your head round this.) Alzheimer's patients often lose out because of this rather arbitrary distinction.
Remember that well managed needs are still needs and must be considered. If your dad needs complex care and you are providing it outside the CHC system, that cannot be used to show his needs provision inherently falls outside the CHC system. Secondly, his needs being met does not mean his needs are not eligible. Assessors often try to establish this (false) position so it's worth saying calmly that "well managed needs are still needs", which is a direct quote from the National Framework (also worth reading). 1 , 2