usually when mine start laying i will introduce another feeder with Layena pellets and give them the option and will start slowly cutting back on flock raiser. some people have mentioned giving oyster shells as a option , but i never have with my ducks and they do fine , layena has all the good stuff they should need.
its good practice to pick the eggs up (usually mid morning ) or whenever they lay and put them in the fridge , at least that is what i do , or throw them straight into the pan and eat up !
if you have trouble getting them to lay in one spot , get a few ceramic eggs and place them in the nesting area and they should develop a habit of laying in one spot.
glad your getting wonderful duck eggs !
as far as shelf life , if i have eggs that are sitting over 3 or 4 weeks , i will hard boil them and they are good for a week in that state (in a sealed container refrigerated ) or use them for baking . good rule is try not eat any after 6 weeks or so.
enjoy !