You mention Layers Mash Paddy - Pellets are better for ducks as mash can get stuck inside their bills when mixed with water.
I feed pellets ad-lib in a hopper and wheat in a half bucket of water.
In the winter when they are not laying, they need very few pellets as they don't need calcium to form egg shells. Drakes will eat more wheat than pellets because they don't lay eggs of course.
If you provide both, they will regulate themselves, taking what they want.
Putting wheat under water softens the wheat but also stops rats, mice, crows and so on from getting it. The water can get muddy but since the wheat sinks, you can drain this off carefully daily and refil with fresh water and top up the wheat.
I'll let Foxy answer the Cod Liver Oil question!