How to properly direct light sample GGX in path tracing
This may simply be a typo or oversight. Note that in the previous section they've been discussing the microsurface BxDFs $f^m_r(\mathbf{i},\mathbf{o},\mathbf{m})$ and $f^m_t(\mathbf{i},\mathbf{o},\mathbf{m})$, which are naturally evaluated at a microsurface normal, but in this section they've moved on to talking about macrosurface BxDFs, which have the microsurface integrated out.

AFAICT, the intent of equation 19 is just to say that the total BSDF is a sum of reflection and transmission parts, so it doesn't really matter ($\mathbf{m}$ is a free variable there and could be named anything), but to be less confusing it should probably have been $\mathbf{n}$. Maybe they just forgot to update it during a round of edits, or something.

Yes, you can evaluate a BRDF/BSDF with any combination of input and output directions and it should give the correct answer.