You can do any of those things. I use a rack and dislike halving slabs so I just bend the ends inward some so that they don't extend to or past the grate edge. For slabs at the edge of the rack where, owing to their proximity to the WSM's curvature, you can place a doubled piece of foilunder the exposed slab bottom and up along the end edge to reflect heat.
Some who use racks prefer to keep the slab(s) in the rack's center whole and halve the slabs that will go in the ends, obviating the need for foil.