Yes
You can state the monthly times 12 there
The form will automatically populate that field
Yes
Yes, to 1 and 2. Consular Officers and NVC understand your SSI is not taxable, and not actually considered. Nothing wrong with stating it, but it will not help. You must also understand that the Consular Officer's decision is about how likely it is your wife will become a public charge. It is not just about whether your niece qualifies on paper. They are trained to consider "the totality of circumstance" which is going to include your wife's employability and ability to support HERSELF. They will want to know your plan for how the two of you will sustain yourselves without her receiving means tested benefits. You joint sponsor needs to know this too, because she's signing a contract obligating HER to repay the taxpayers for any of those benefits. This contract does not have a clear expiration date. Yes, there are ways for the obligation to expire, but there is no guarantee, any of those things will happen, potentially obligating the joint sponsor for the rest of their life.
You say you have a special need for your wife to be with you. If that need means she won't be working, you'll be facing a potential denial, no matter what your niece's circumstances are.