If the Bluetooth on a participant's iPhone turns off, will BACtrack Skyn still collect the data?

Yes. BACtrack Skyn is collecting data at all times when the device is powered on. The Skyn hardware can store approximately 72 hours worth of data. As long as they turn on their Bluetooth and appropriately connect their Skyn with their app, the data will still sync and upload. But, if after 72 hours they do not connect to the app, the oldest data points will be overwritten by the new data coming in.