Cortel Steel Panels

Mortar or thin-set will not adhere at all to metal of any kind so there is no sense in even considering them. Do you have access to a welder or a friend who welds? If you welded pins to the back of the sheets and then drilled holes into the wall for them to go into you could epoxy the pins in and you would have a very secure attachment. You could certainly try construction adhesive but over time exposed to weather and temperature changes it is going to weaken.

How large is that feature? Is that a reference on the drawings it's hard to tell.

Corten does a better patina. I've seen it and mild steel side-by-side and certainly would prefer it. It's considerably more expensive considering you won't be using much and it's decorative, it's probably a better choice. As Bondo mentioned, the front side discoloration can usually be removed so that the entire surface will oxidize evenly.