MiraNYC

From my experience in NC and MA DMV's, they treat the extension letter as to extend one year on the Green Card expropriation date. I also traveled overseas (not to my original country) 4 times with the extension letter, I had no problems at all. The first time I traveled, I did ask the custom officer about the letter, he told me it means one more year on the Green Card expiration date. 
Personal experience.

I just got approved, and I could have applied for N400 on June 10. If I was you I'd wait, I-751 takes one year anyway, it took me 352 days to get approved. Just try to be patient, you're very close!

Hi,
Personally I think your best option is to look for another country to build your dream in. The fact that you're from Spain gives you way more choices than 90% of the people on this site.
Ok, back to your question. Your husband is still on an F1 visa? And he's been a student here fore 4-5 years? May I ask what sort of degree he's studying for? You have to be aware that for the past couple years, there were too many applicants for H1b visa and the result was that now the application would go through a lottery process. If your husband has an advanced degree, such as a master's or above, the chance for him to get selected will be higher.
And then, I'm not sure if you know this, but you can only apply for an H1b starting April 1 (unless you're working for a non-profit), and without an OPT, you would have to wait til Oct 1 to be authorized to work. Ok, so from today til Oct 1, 2015, your husband is not allowed to work. Do you think you can handle this with your financial issues?
Btw, your husband is totally allowed to apply for an H1b, but it has to be in the same field as what his degree is in.
A lot of people spend lots and lots of money to try to live and have a life in the USA, while I feel your pain, I'm sorry to say that sometimes it just doesn't work out for everyone.
I personally came here on a student visa and obtained my master's degree at a top 30 university here, after graduation, I still had a hard time getting a good job. Eventually I was lucky enough to find an employer willing to sponsor me, and then was approved for my H1b visa. However, pay attention to the word "lucky", I have seen too many of my classmates who have been struggling to find employment and sponsorship, and in the end chose to return to their home country or seek other opportunities somewhere else.
I know a lot of people have an American Dream, but the truth is, sometimes dreams don't come true. And who says that you can only have one dream? I'm saying this from the bottom of my heart, think about your children. What kind of future do you want for them? It's unfair to have them bear the name of "illegal immigrants." Anyway, from the way I see it, I don't think there's an immediate solution to your visa problem.
Hope this helps.

I was in the US for eight years in what you call "limbo". It's called temporary status in the real world. It's temporary, when it ends, you leave.
I had a job, a driver's license, car, all my stuff, my whole life here, and yet there was a part of me that was always prepared for it all to end at any time, because that's the reality of the situation. I didn't buy property even though I could afford to and wanted to because I knew I was in the US temporarily and it would be too complicated to leave if I had even more ties. I put my whole life on hold. It sucks, I know, but it's reality, and if you can't face it, this game isn't for you.
And you're a citizen of the EU, a privilege I would have LOVED to have during this whole time, instead if I had gone home, I would have to go back to an actual third world country, not Spain. Or literally any other EU country where you and your family are free to legally live and work. Cry me a river.