yet-another-intercom-hack

it's ok to have Bat+V to battery +V, Gnd to battery Gnd, and be connected to USB at the same time ( for Rx /Tx ) ?

Yes, that's fine - the Pico has circuitry in there to auto-switch between power sources (if you have instability you could actually remove that diode, but either way nothing will get fried :) ).

For the LiPo batteries, make sure you've got one with protection (a little PCB on the battery - most of them have this now). It restricts the power that can be drawn from the battery and makes everything a lot more safe :)

You could also use a breadboard power supply?

is the following actually doing something or just a nonsense ? ^^
// ( .. ) if ( at.isBusy() ) return cb; // ( .. )

Personally I think it's nonsense. isBusy will always return true if called from that callback I think.