You can certainly wear a nice dress, but if it looks like a wedding dress, it may cause some difficulty or put in an awkward situation. We had a wonderful experience with them for our dessert party. We did not say we were getting married, we simply just told them it was a holiday get together (we were married the week before Christmas). We didn't want to risk running into any trouble. We had our dessert party as an event for our guests the night before our wedding, and it turned out to be perfect timing scheduling it this way. It was a wonderful way to relax the night before our wedding & our guests thought it was a great way to kick off the festivities
I don't know anything about having a cake at a DP. If it is allowed, I"m going to guess it would need to be one catered through Disney (not an outside vendor) & it cannot be a wedding cake.