Hello.
24V is well beyond the recommended 12V maximum for powering the Arduino UNO and is right at the upper limit of the shield. You should probably use a lower-voltage power supply, especially if you want to use it to power both the shield and the Arduino. As for having the power connected through the shield while programming, (assuming it is an appropriate voltage) that is the same as powering the Arduino through its DC power jack while USB is connected. Arduino intends for this to work, but we generally do not recommend doing so for the reasons mentioned in this forum thread.
- Grant