To answer pretty quickly:
1. You didn't have at the beginning to send any original to the USCIS. The instructions especify that you can send photocopies, but well if you did so now for your health and stress is better to translate all the documents that are in Spanish. How many documents you must translate? It is my understanding that should be just your wife birth certificate and your marriage certificate. How much you pay for the translations?
You need to carry your own documents, is like your own packet and you must carry another one with originals and photocopies for you for your records and other set for the officer. If they want again another document they can take it from it.
You must make copies of your tax return, you must bring it with you. Make a search around this forum there are many other things to bring. Make a checklist. Actually the interview notice give you a checklist.
2. Not always, but they can keep a couple or one picture of you, that depend on the officer at the interview. You don't have to make a copy of your pictures. If they want one, just give it to them okay? You must bring your photo albums or loose pictures but in order so if they ask and even if they don't just show them in order.
When I said all the documents are her documents, she is the one who is the adjustee not you. Just in case you should bring your US passport or birth certificate plus both will need an ID card issue by the goverment to enter to the interview, she must bring all her EAD cards.
3. Uhmm, well never is late I think. Yes add her to all the bills and accounts possible. If you don't have the time to wait for the statement, ask your companies if they can give you a letter that states that she is an authorize user or is a joint account holder. Remember love is not enough.
4. Your child's passport won't work. You don't have to demostrate that he is a US citizen, you must demostrate that he is the son of you and your wife, so you need his birth certificate that show both of your names on it. You can bring the passport as extra but as I said bring the BC that is a must.
3. After the interview approval and depends on the district office your wife's passport could be stamp. The officer at the interview will tell you how long will take to receive the actual card can go from 2 weeks to 2 months approx. To travel outside the US she needs or the stamp on her passport or receive the actual card on the mail but for Xtmas she should be a permanent resident.
Please make a search on this forum by "Interview experience" or "what to bring to the interview". Go back like 2 or 3 pages and search the several post under those phrases you will find a lot of info.
Good luck,