American Citizenship - Spanish Citizenship

Hello,

So my basic question is: How does an American spouse apply for Spanish Citizenship, while the Italian Spouse applies for American citizenship, at the same time?

Details: My boyfriend and I are planing on getting married in Spain. I am American, he is Italian. We are getting married in Spain because I will open another branch of my business in Spain, where we both will live and work for the next 5 or so years. We, chose Spain because it recognizes same-sex marriages, whilst Italy does not. So I'm wondering exactly how this is going to work as far as citizenship for us both. He has Italian citizenship obviously, which I won't be applying for, but we will be living in Spain, where he does not have citizenship, technically speaking, and does not need it. So will I still be applying for citizenship by marriage, or could I apply after 5 years or so living and doing business in the country? The only reason I'm wanting to acquire citizenship in Spain is primarily for the EU passport. I figured since I'll be putting in many years in the country and paying it taxes, if there was a way to get the passport, then I should be advantageous about it. Also, it just makes traveling easier, and potential retirement locations/countries more broad.

Moving right along, after those 5 years are up, we will return to America to continue the next phase of our business and will settle down there for unforeseen amount of years. From what I gather, for him to be granted his green card or citizenship, we would have to have been married for 3-5 years (I keep receiving different answers on the time-frame)? Also, a certain amount of that time would have needed to be lived in the united states, correct? Would we also need to have been married in America, or do we just need to apply for a marriage license in America, once we have been married in Spain?

So, in my mind I'm thinking it would go something like this. Using the following example, assuming that the time required for him to receive citizenship is 5 years, and 1/2 of that time(2.5 years) must have been lived in the States:

Get married in Spain 01/01/2015. Apply for marriage license around the same time (which i'm assuming has to be done in person) in the States after showing proof of our marriage in Spain (California to be specific). He receives his residence and work permit.
01/01/2020 we move to Los Angeles. Sometime around July of 2022, he can apply for his green card/citizenship.

Is that how this whole thing would play out, or I am completely wrong? Has had a similar situation of both spouses applying for citizenship for different countries? I know this is a lot of information and that I have asked many questions. I just been researching constantly and I can't do it anymore. I rather hear from some people who have done this before or has some real knowledge about this topic and can streamline this for me. I sincerely appreciate any answers you can provide me. Thank you!