JOAE

They were correct.  If you didn't have enough income to file, then you just indicate that your income didn't require filing....either by a letter or by writing that on the I-864 space where it asks for that information.

You need to file at least the last 3 years ... or have a valid explanation for not filing ... valid means one that the IRS accepts ... “ no way of proving income “ , “ my mother is an entrepreneur “ and “I live in El Salvador “ are not reasons acceptable to the IRS 

All US citizens are required to report income world-wide if it above the minimum required.......no matter where he/she resides......

Correct that you as primary sponsor will declare a household size of 4... but if you are using a joint sponsor they need to add only your husband to their household number 

You will be your husband's primary sponsor.  You must provide evidence of domicile in the US.  You must have documented current income (continuing after relocating to the US) to sponsor a family of 4, or a qualified joint sponsor for your husband.  US citizens do not require sponsors.

Only your husband will need a financial sponsor ... assuming you are also a USC

All. You will always be the primary sponsor. So your income, or joint sponsors will have to make enough to cover a family of 4, to include yourself.

Sounds like You will definitely need a joint sponsor (you are your husband's primary sponsor) who will also sign an I-864 (You will also sign one)....and your taxes must be in order (or have an explanation as to why you were not required to file).

co sponsors are for non citizens