"John Harper" writes:

My understanding is that the only difference between
APP and NAV modes is whether the GS coupling
is enabled. If the GS and dest waypoint happen to be in much
the same direction you could get a long way down...

Your understanding is incorrect.

One of the differences between NAV and APR mode is how aggressive the
a/p will be to keep the needle centered.

The GS capture is not armed unless you have a localizer frequency
channeled in (ie, tuned in a nav radio, and the a/p is coupled to
that radio), among other things.

-jav