Just to be sure you can test it if you switch your phone to flightmode.

I haven’t activated my pass yet, only added journeys to my Trip. If I activate my pass and add a journey to my pass with my phone in airplane mode and it works without internet access can I remove the journey without using a travel. day?

Hello Edward You need internet access to activate a travel day and show your QR code, otherwise it just won't open and you might get fined. You can add and remove as many journeys as you want, you don't have to activate a travel day for that. If, however, you have already activated a travel day for the same day, adding or removing journeys won't cancel it. You can only deactivate a travel day before midnight of the previous day!