Can I avoid overcooking my candied lemon decorations?
Try gluing them to the cooked cake with a sugar syrup or caramel.  Caramel is quite sticky, so it should hold the candied citrus on the cake with no problems, at least during transport.  How hard your caramel is will probably affect how well it cuts, but I think a somewhat softer caramel (or a thicker sugar syrup) would probably cut just fine with a reasonably sharp knife.

Alternatively, you could always try standing them up vertically in the cake, depending on how stiff your slices are and whether that's a look you're interested in for the assembled cake.