icons – Up arrow for increase, Down arrow for decrease, what for no change?

Because you are using arrows to indicate up and down, keep the iconography consistent, and use an arrow for no change – pointing sideways, either to the figure, or a double headed arrow pointing both ways.

Mixing the images would be confusing, so keep it consistent. Colour it black or grey – something to indicate neutrality, and the up ones green an the down ones red (or vv if that makes more sense). This means that for most people, they can look at the screen and identify the predominant colour showing easily, and get a very quick overview. Keeping the “no change” neutral means it doesn’t interfere with this.