[Brightcolours]

When I do focus and recompose, I tend to not put a subject which is 1.5 meters away in the corner of 85mm f1.8 images.... A bit of a stretch as example.

With subjects in corners, usually it is in a bigger scene (wider angle and/or further away).

In the portrait lens example, you do not put the eye in the corner of the image, but perhaps just outside the limited AF area of your FF DSLR. In that situation, the outer AF point does not quite reach the subject. Selecting the outer AF point closest and then recomposing in a further than 1.5m example will result in a much smaller angle of rotation than your " center point to corner at 1.5m" scenario, and will get the subject into focus.

You have a valid point where field curvature is concerned. With a lens with strong field curvature, focus and recompose is a recipe for disaster (Hello, Canon FL 55mm f1.2). You can't rely on AF points in such a situation, and have to judge focus in a different manner (without recomposing).