Julie,
Without wishing to add to your problems:
There is a difference between "Advocate" and "Independent Mental Capacity Advocate (IMCA)" , as in Sandy's post.
The two are not the same; the two cannot be called upon in the same circumstances.
If you, Julie, are willing and able and involved in the life of your Mum, as you are, then an Advocate can be involved, but not an IMCA. (They may be the one and same human being, in their own 'reality', in their daily dealings with people who need an advocate, but not always. And their responsibilities and involvement are totally different, if that person being 'assessed' for whatever has no family/friend members able and willing and involved enough to speak on their behalf.)
http://www.publicguardian.gov.uk/docs/making-decisions-opg606-1207.pdf
There is only one pot of gold for the SS. But there are different priorities within their budgeting systems, as in any other business. That should not concern you, Julie. You follow your Mum's needs and your own ability to know your Mum's need, not the budgets.
I don't recommend that you get other residents to support your cause. The manager, by all means, because it is the manager who wears the Overalls!! Mr/Mrs Overall bears the responsibility for whatever happens in the sh/housing. But not other residents.
Hope you find a way through this all.
Forgot to say that the Advocate is there to support your Mum. Not you. That doesn't mean that the Advocate will be automatically agin you - far from it. S/he will appreciate and expect your own input, otherwise the Advocate will not be listening to those who know best about their relative. So, you may not get to meet/speak/brief the Advocate in advance of the meeting, but that should not be a problem.
Did the SW explain to you whey they felt it necessary to appoint an Advocate? That would have been proper procedure. Is there a history of conflict? If not, then I would ask to know why the SW felt it appropriate to call in an Advocate.