Can I apply for my mom to get green card while I am a green card holder?

You need to be a US citizen before you are eligible to file for your parents.

I am an LPR and am petitioning for my husband who does not live in the US . Someone told me I might need a joint sponsor ....

If your income is not sufficient you need to have a co-sponsor. See www.uscis.gov and search for poverty guideline.

When would I need a Co-sponsor or Joint sponsor for I864 or I864 EZ? I don't know which form to fill when we have a co-sponsor

Best to consult with immigration lawyer. But as a co-sponsor you may file I-864a.

You need a signed copy or go to IRS and asked for a stamped copy. You may send a unsigned one and take a stamped copy to the interview.

USC petitioner;s background is not in question. The short answer is that your fiance can apply for you.

I am a green card holder, can I apply my mother for green card to come in the US or do I have to wait on my citizenship?

As a LPR you are not permitted to file for your parents. Wait until you are a USC.

Affidavit of support 864 form. Tax return as proof of income

You only need 2014 tax information and w-2 including letter of employment. If electronically filed, go to IRS office and get a stamped copy.

I am petitioning for my husband. Can I file i-864 EZ if I am not the only sponsor for financial support ?

if your income is sufficient file I-864 and if not find a co-sponsor.

Will I get the tax change reimbursed in my next paycheck?

Look this is an Immigration law forum not tax. You need to contact a tax lawyer or accountant.

It does not matter that you overstays your visa at one point and another. The fact is you should consult wit an immigration attorney and he or she...