1. The Drive Bar shows how much Drive Sora has and it is yellow. When you use Drive Moves, it takes up the Drive Bar. When Sora fuses with Donald or Goofy the Drive Bar changes to the Form Bar (it says "Form" when it use to say "Drive) which is grey and shows how much longer Sora can stay fused with that person (it appears to always be set to 5 bars). When you attack or use Special moves while in Drive Mode it takes up the Bar. That's why Sora's Drive Bar appears to get bigger.
2. Donald's picture reappears right before they switch scenes because right before it does, the selecter on the Command Menu goes to the bottom slot, which allows you to use Drive Modes. If you are already fused, it must allow you to unfuse. That's why Donald's status bar reappears right before the next scene switch.
The second one is only my oppinion, but I'm pretty sure it is correct.