What details in Part III suggest that Farquhar's journey occurs in his mind?

To me, there are a few major things that suggest that this
journey is not truly taking place.

First, he seems too
aware for it to be real.  He is, for example, able to see the bullets that have hit the
water and are drifting down towards where he is.  That seems
unrealistic.

Second, his consciousness seems sort of
weird.  He sees his own hands as separate from himself.  He sees this vortex of color
where everything (trees, sky, etc) is indistinct.

Finally,
the soldiers' actions seem unlikely.  This is especially true of the cannon.  It seems
so unlikely that they would fire a cannonball (not very accurate) at him.  And then it
seems unlikely that they would miss if they used grapeshot.