I really don't see the problem here.
Ny is computed from an accelerometer mounted near the top of the rear bulkhead.
If it is to be used to estimate sideslip then the reading needs to be corrected for rudder sideforce (deflection) roll and yaw rates. If airspeed information goes south then so does the estimated sideslip.
So far as I can see, the only use of Ny and/or sideslip in the EFCS is a part of the lateral turbulence alleviation function.
In Alt2 there is no bank protection (and therefore no spiral stability term). Consequently the EFCS neither needs nor uses either Ny or estimated sideslip for roll control in this mode.
In Alt2 the rudder deflection is controlled by pedal angle and yaw damper commands. The yaw damper uses yaw rate; there is no Ny or sideslip term.
Consequently the EFCS doesn't use Ny at all when Alt2 is engaged.
The estimated sideslip is sent to the PFD.
That's it. Full stop.