Hi. Can answer only part of your question, but here it goes. Not very likely that you will be deported. If the Spanish government were so efficient in that sense, half of Spain would be empty. If you want to have peace of mind and have a lot of patience, you can apply for a visa to get married. Unfortunately, I don't know how it works. You'll need to contact the Spanish consulate in your area.
As to the requirements, best thing to do is to go to the Registry and ask for the requirements. They change depending on the area. It's much easier to marry where your boyfriend is registered (empadronado).
That won't necessarily give you all the information you need. I am trying to get married in Spain myself and it's chaotic, to say the least. The information we have been given is contradictory and we are not getting much help. On top of that, they seem to have been on strike for months now. I actually joined this forum in search for help and information. You can have a look at my message, to see if it's any help to you. If you find out anything useful, could you let me know? Every little helps. Good luck.