Hi Alex, I hope you're doing well!
1.- It is recommended to use external oscillators for the QN908x, as they may provide additional accuracy and precision, which are needed for the Bluetooth Low Energy protocol. Some example parts and a BoM, as well as schematics can be found in the QN9080DK's Design Files archive, which can be found in the following page of our website.
Also, could you please take a look at the Hardware Design Considerations document for some more specific information about the different available clock sources for this MCU? Specifically, chapter 5 provides some very relevant information.
2.- The 3.5 mA value is described as the typical current consumption on the device at a temperature of 25°C. However, these typicial values are not guaranteed. The 3.5 mA current consumption value, for Tx at 0 dBm, has the following foot-notes attached to it:
 Typical ratings are not guaranteed. Typical values listed are at room temperature (25 °C).
 Characterized through bench measurements using typical samples, with 50 ohm loading on RF port.
Thermal conditions, as well as other factors can cause this power consumption to change.
Please let me know if you need any more information.
Take care, best regards,