Serial Peripheral with MDBT42Q

Will it be possible in JS code to claim the UART

Yes, just move the console out the way. Bluetooth.setConsole(1) is fine for this.

I assume the RN2483 isn't going to boot up with TX high (but that could be a gross presumption)

It probably will do, but that's no problem.

Can the BLE radio be disabled in code?

Yes, NRF.sleep - but it uses so little power it makes sense to keep it on

What's the deep sleep current like on the MDBT42Q

20uA when advertising, 10uA or under when asleep on the breakout board. On the module itself (without the voltage regulator) you're looking at 3uA or less.