When I was there (about a week and a half ago), Mickey & Minnie did come out in their Diamond costumes late in the day. From what I've heard, you can expect them in Diamond costumes any time after Soundsational. On one night I was able to meet them both together. We got in line around 6pm, the line was LONG (over an hour wait) but they did not cut the line and guaranteed if you were currently in line that you would get to meet them. I also noticed they also came out closer to parade and closing time (I would check the app for times). One night after the 2nd PTN parade, a lot of people were waiting (in an organized line) and a CM came out and said that Mickey & Minnie would be coming out but meeting separately and that basically you were waiting in line for nothing and when the characters came out you would have to follow them to their line. Pluto also came out on this same evening. I also saw Goofy come out on other occasions.
It's a little confusing and messy, and CMs are very mysterious about it in my opinion.
I hope you have luck in seeing them!