1) original certified copy - no, unless they specifically ask for it. If needed you can send it to them after the interview
2)I-797 - no, they know this anyways.
3) Scans are accepted and USCIS explicitly points out that they do unless they specify otherwise
4) my experience is that there are no original documents needed, only copies. I brought my original documents with me just in case. They never asked for it.
5) Letter of intent to marry as you indicated it is fine. They should already have that anyway because it's part of your I-129F submission (which they receive at the embassy)
Long story short: You're fine, no need to freak out ;).
Ensure that your partner reads and knows exactly what to bring because it sounds like you're just throwing absolutely everything and the kitchen sink into a bag to bring which really isn't necessary. The only original documents or copies of documents they are going to need should pertain to your partner (so it should be easy for them to bring those originals).