If you're not actually going over the hips or bust (thus the lateral deformation is limited to one arc) you can pretty much use straight steel for everything.
But I think you're saying you DON'T want to tightlace with this, that it's just a costume piece ?
In that case, I would ditch steel entirely - just use heavy duty cable ties or reed or something
The shaping come from the seams, anyway - not from the type of steel in the channels - the steel is just there to make sure the fabric holds its shape instead of deforming.
And yes, to the question at the end, you can't really make a corset that can be worn both ways unless it's a really generic and symmetrical shape - which won't be comfortable for actual tightlacing. A corset for much waist reduction needs to be shaped to the body instead of just being a tube with flares on the ends. Does that make sense ? In the one that is shape, if you were to let it be looser or tighter it would deform from the ideal shape it was created to attain and smoosh you weirdly or have extra fabric wrinkling when it's displaced from where it ideally would be in relation to the body.