When I originally read your message i was thinking that you wanted to work in spain first and then apply for citizenship. I have also applied for citizenship but also while in spain so I don’t know if the required paperwork is diffferent if you apply from the u.s. I wouldn’t be suprised if it was.
if i remember correctly, for nationality you need all the same stuff as for residency minus the police report but they also asked for the marriage certificate of my parents and my certificado de empadronamiento You can get more info from the ministry of justice website http://www.mjusticia.es but the best advice i can offer you from personal experience is find out what the office you are actually going to deal with wants you to provide. My experience was with making the application here in spain so the process i went through was probably more difficult because there are so may people try to immigrate.
an apostille is an official seal they use for documents which will be used internationally. It gets attached to the original document.
the police report is just a criminal history report. you can get it usually from your local police dept or at least make the application for it from your local station.
dual nationality between u.s. and spain is a don’t ask don’t tell situation as i understand it. the last time i checked u.s. law says something to the effect that just because you declare loyalty to spain doesn’t automatically mean that you want to lose your u.s. nationality so your not going to suddenly lose your u.s. citizenship and two countries don’t share this info but i’m not a lawyer and i’m just repeating what i came across when researching the subject myself. definitely do some research on your own as well.
If you have any other questions ask and if anyone else reading this has heard differently please share the info.