A) you keep on using the word "almanac". That term only refers to the imprecise information about all satellites that every satellite broadcasts, mostly to improve TTFF. The actual position used for navigation is called "ephemeris", and each satellite only broadcasts its own.

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).