Spoofing of Galileo was possible as long as the authentification was not enabled. https://www.septentrio.com/en/learn-more/insights/osnma-late...
B) none of that other stuff in the navigation message changes the pseudorange, which is what spoofers mess with. For a networking analogy - pseudoranges are calculated based on layer 1/2 properties of the network. (Specifically the code phase and Doppler shift.) Navigation messages are layer 7 information passed on top of that physical layer. You can change the timing and frequency characteristics of the PRN code without touching a single bit of the navigation message.)
The G/NAV message (note the G - government) is for a separate service - not OSNMA - where not only is the navigation message encrypted, but the PRN code is also encrypted (symmetrically, so it can't be done for the mass market or even untrusted commercial customers).
If someone can re-broadcast the keystream and control the latency I perceive as a receiver, how would me checking that the MAC is correct help?