When flying the Mavic should I first turn off all Bluetooth devices in the immediate vacinity?

No you dont need to. I wear my Bluetooth watch connected to my phone while flying.

Should not be necessary - but Bluetooth occupies the same 2.4ghz radio frequency space as the RC/phone/drone data comms. So I'd would turn it off if you are calibrating any of the devices in close proximity - as there can potentially be harmonics that just may interfere with the process.

It's advisable but not essential. The less interference the less chance you will experience issues with the radio transmission from video lag to signal loss.

No you should be fine I have a bletooth watch on all the time and it's fine

In most cases, you don't have to turn it off. In my personal experience, it would be better if turn off the Bluetooth devices especially when flying a Spark (as I saw you might have one), it might cause the laggy issue of the video transmission.