@ddcontrol This sounds like your letter of intent was on the shorter side. I was recently approved and both of our letters were 2-3 pages. We explained how we met, activities that led to falling in love, what we did in between like vacations and activities with family, and then closed with statements that said we wanted to spend our lives together and what that entailed.
As far as what language for the supporting affidavits from family and friends, it needs to be in English and they need to sign it. My fiance's sister typed hers using translator, signed it, and emailed it to me.
With evidence, you give them so much that there is no question that it is legitimate. I also had 4x6 pics developed with 6 pics on each 4x6 and included 5 of those. I didn't include our communication messages because I felt that was a bit private but I did print out our relationship status update on FB and included that. I also kept our engagement post public so it was accessibly for them if they peaked.
Reading the list of items that count as evidence they sent you, do you have any social media posts about your engagement? Did your fiance take a picture of her ring and post it? You could include how you proposed. Yes, a letter from her parents or any family and friends would be very helpful too. People that have seen you two together and can honestly say, they look at each other lovingly and truly care for each other, or something similar that shows you two truly want to be together forever. I would recommend gathering as many of their list provided and satisfy them the best you can.