The switch is easy, just set it up to aux 2 and it should switch modes. Also, on your BetaFlight it should detect it, if it doesn’t you need to install a special driver. There is a Joshua Bardwell video on it.
That is Voltage sag, it’s normal to have a certain level of it. On my betafpv meteor 65 I try not to let it get below 3.2 with sag, although I have gone lower and been fine. However that is 1s only whoop, the mobula might be different with 2s.