Wedding in Spain for Two people living in U.S. (one is EU citizen)

I know this is a question probably best asked to the Spanish Embassy here in the States, but I am having a difficult times contacting them. I have a question concerning wanting to get married in Spain. My fiancé is a duel U.S./British citizen who has family in both England and Spain. Her mom in London cannot travel to stateside due to medical issues. However, she can travel by train to Spain. We wanted to get married in Madrid, but after reading the information on your website, we still have some questions. First, is it possible for two non-Spanish citizens to get married in Spain if not previously having residency for two years? If one of them is from the EU, then we think it’s possible. However, you would have to confirm with the Spanish authorities, and specifically with the Registro Civil where you would be getting married.

Second, the website states that it will take approximately 21 days to get a marriage license in Spain. However, is it possible to turn in the necessary paperwork through mail, or do you have to be there in person to turn it in. Thanks in advance for the help. We think that you would have to do this in person, however again, you would have to confirm with the REGISTRO CIVIL directly.

Third, would I and any guests need visas to enter Spain if we are planning on staying around 3-4 days. Thanks. US citizens do not need a visa if their stay is for less than 90 days.