SabrinaJerome

You cannot file back taxes electronically because the form changes each year, and the IRS requires taxes to be filed on the respective years form. You will need to go to the IRS website, download the appropriate form, fill it in by hand, and mail in your taxes to the address listed on the form. Any tax preparation site will say the same thing. Some local tax preparation companies like H&R Block can help you prepare the past tax returns, but they will give you everything in an envelope for you to mail in on your own, because they cannot be filed electronically.

You need 3X the 125% of poverty, or about $60k in easily cash-convertible assets. It seems these are defined as assets that could be converted to cash within one year. An account like a 401K for instance is considered eligible.
You need a tax return or a statement declaring that you did not have to file taxes due to not enough income.
The AOS form lists either assets or income, I believe that implies that it applies to anyone and I don't believe it's embassy-specific. What IS embassy-specific, or so I read, is assets for a K1 I-130 are embassy specific. Doesn't apply to married couples.
They spend a little time looking at the sponsor in terms of intent to reside, it seemed to help us that our assets were already parked in a US bank
Good luck

Each sponsor needs to submit taxes and W2s if he or she is not using an IRS printout, or provide a reason for not filing taxes.
That's from
http://www.uscis.gov/forms/tips-filing-form-i-864-affidavit-support-under-section-213a-ina

If you have earned below the filing requirement for a given tax year you would submit a signed statement to that effect.
If you earned more than the filing requirement (worldwide) then you would need to submit the most recent tax year's return or IRS transcript and be prepared to show the 2 years prior to that (if requested by the consular officer).

Correct, attach the signed letter to the I-864. Some folks have just written the statement on the I-864 itself. I would opt for attaching the statement/letter.

1. there is no DCF option for Spain, sorry.
2. get married and file the I-130 showing the foreign address in Spain. Why? it jumps the casefile into the 'filed from abroad' queue, sort of an auto-expedite adjudication queue, and ya save some time. when it gets to NVC, you have to prove up usa domicile, but I think you've got that handled.
3. conditional green card 'substitute'? ain't no such thing with that label, sorry. maybe yer confusing an endorsed CR-1 or IR-1 visa, where the human is granted a temp I-551 document at the airport with a duration of 1 year? This is normal, as awaiting the actual green card delivery takes some time. So how does this pertain to your question about EAD and AP? A Human with an endorsed immigrant visa does not need EAD or AP at all - in fact once the VISA is endorsed at the Port of Entry into the USA, that human can turn around same day and return to Spain.
4. I say get married in Spain asap and file the I-130. why? you have some idea of him going back in the fall, and I'm sorry to say - it takes time to get a spousal immigrant visa - there's 3 phases on it - and any phase you can shave time on is a blessing.

Yes. Just this ^