Alright, i have gotten the binary forms part of the story. I may or may not ever know the field side of things, that's life.


From experiments with the website http://www.numbertheory.org/php/classnopos.html and comparison with my own programs, it appeared that the division in half amounted to identifying the distinct forms (whenever the principal form does not represent $-1$)
$$ \langle \alpha, \beta, -\gamma \rangle \; \; \mbox{with} \; \; \langle -\alpha, \beta, \gamma \rangle,   $$ where these are ``reduced'' in the sense of Gauss and Lagrange when
$$   \alpha \gamma > 0 \; \; \mbox{and} \; \; \beta > |\alpha - \gamma|.  $$

So it is needed to show that these really are distinct classes when $1$ and $-1$ are distinct as forms. However, this is not hard. If the two forms above are equivalent, then the opposite of $\langle \alpha, \beta, -\gamma \rangle$ in the form class group is $ \langle \gamma, \beta, -\alpha \rangle. $ Thus the hypothesis amounts to
$$ 1 =\langle \alpha, \beta, -\gamma \rangle \circ \langle \gamma, \beta, -\alpha \rangle.  $$ We are allowed to insist that $\gcd(\alpha,\beta ) = 1,$  important. Can always be arranged, although the result may not be ``reduced'' any longer.

The algorithm of Shanks, Buell *Binary Quadratic Forms*, pages 64-65, tells us that, with   $\gcd(\alpha,\beta ) = 1,$
$$ \langle \alpha, \beta, -\gamma \rangle \circ \langle \gamma, \beta, -\alpha \rangle = \langle \alpha \gamma, \beta, -1 \rangle.  $$
In short, the hypothesis that $ \langle \alpha, \beta, -\gamma \rangle \; \; \mbox{and} \; \; \langle -\alpha, \beta, \gamma \rangle   $ are equivalent implies directly that the principal form represents $-1.$ Very satisfying from my point of view. Note that we do not need to use Shanks, various books discuss the ``united forms'' approach of **Dirichlet**, pages 55-57 in Buell. On page 57, he confirms, with $\color{green}{\gcd(a_1,a_2,B)= 1},$ that
$$ \langle a_1, B, a_2C \rangle \circ \langle a_2, B, a_1C \rangle = \langle a_1 a_2,B, C \rangle.  $$ Dirichlet gives the same outcome as the Shanks method, but with no additional $\gcd$ assumptions, using $$ a_1 = \alpha, a_2 = \gamma, B = \beta, C = -1.  $$
This is also Theorem 98 on page 138 of Leonard Eugene Dickson, *Introduction to the Theory of Numbers*. 

Put another way, when the principal form does not represent $1,$ we get a distinct form that does represent $-1,$ and Dirichlet says 
$$\color{magenta}{ \langle \alpha, \beta, -\gamma \rangle \circ \langle -1, \beta, \alpha \gamma\rangle = \langle -1, \beta, \alpha \gamma\rangle \circ  \langle \alpha, \beta, -\gamma \rangle = \langle -\alpha , \beta, \gamma \rangle}.  $$ As $\langle -1, \beta, \alpha \gamma\rangle$ is not the principal class, the result of the Gauss composition gives a different class from the original, therefore  $\langle \alpha, \beta, -\gamma \rangle$ and $\langle -\alpha , \beta, \gamma \rangle$ must be distinct.