It appears that, for me, I can play ‘offline’ only by going into options>data and turning online functionality to ‘off’.
In order to run MSFS (and avoid having it crash to desktop) it appears that throughout the whole process I still need to maintain an active internet connection. If I have no internet connection I cannot run MSFS as it throws up an authentication error. If I disconnect the internet connection, while MSFS is running, then the program crashes to desktop.
Setting MSFS to offline mode and exiting the sim, disconnecting the internet and trying to re-run MSFS leads to an authentication error and will not let me run the program.