My understanding is that Social Services will not control the Deputyship. this is set up for the individual, in your case your husband, and will likely be a solicitors who run it. The lady I type for visits on behalf of the Deputyship to ensure that clients funds are being used appropriately and the client is receiving the care they need and gives the Deputy accountability. The Deputy are usually solicitors but you could also be the Deputy. You could also be a joint Deputy.
I don't have a lot of first hand experience about this, my parents had LPA, only that I have typed reports for 8 years for a Court of protection of office who visits client on behalf of the Deputy. I have never typed a report where Social services have run the Deptyship. they are definitely outside of SS.
I don't know what exactly what happens about your joint account, it does seem to depend on capacity how much control a client has over their own money and I don't have lots of knowledge here but some reports I type a client has a bank card and may still make some decisions but others have no control. Not all clients are dementia, some that I type for have other disabilities.
I think it will vary from case to case, and will take into account, dependents and any debts or liabilities. eg mortgages or property as well as savings. if you share a home together and meet certain criteria the home is disregarded when calculating the value of assets and savings.
From personal experience, depending upon your financial situation and how the care cost are being met, monthly income is used to cover the cost of care. So your husbands pension may be used for that.
My parents owned half of the property we all lived in for many years and I have just had the most horrible year resolving this with social services so that I could stay in my home. The outcome is that my parents income, pension etc is used toward their care fees as their saving are below the minimum threshold. The minimum threshold is approx £25,000. They can keep this amount of savings and when they get to £25k social services will fund the care. The contribution to care is any monthly income.
If you do need a solicitor I can recommend a lady who specialises in care fees. She was amazing, kind and knowledgeable. I would have probably lost my home without her.
I really hope this helps in some way. I feel so much for you as I know how stressful it is coping with all the emotional side of dementia and then this whole learning curve of the legalities on top. It doesn't feel like a care system at all. If there is anything else you need to know I can ask the lady I type for regarding specific questions.