Combine currants, sultanas, raisins, and glace cherries in a bowl; cover with orange juice and soak overnight or up to 3 days, stirring daily.
Preheat oven to 275°F (135°C) and line a 9-inch cake pan with parchment paper.
Cream butter and brown sugar until fluffy.
Add eggs one at a time, then molasses, chopped almonds, orange and lemon zest, and mixed spice; mix until combined.
Fold in flour then drained fruit mixture gently.
Pour batter into prepared pan; level top with slight concave shape.
Bake for 4 to 4½ hours, covering with foil if browning too fast, until skewer comes out clean.
Cool cake in pan on wire rack.
Remove cake from pan and poke holes on surface; feed with orange juice slowly.
Wrap cake in parchment and foil; store in a cool place, feed occasionally with orange juice for up to 3 months.
For covering, mix apricot preserves with warm water, strain and brush over cake.
Roll out marzipan 2 inches larger than cake; cover cake smoothly and trim excess.
Let marzipan set for a few days before icing.
Whisk egg whites until frothy; gradually add confectioners sugar and cream of tartar on low speed.
Slowly drizzle in lemon juice while mixing to form royal icing.