Yes, swan parents know when the eggs are viable. Incubation begins after the last egg is laid. So, a human count of when incubation begins is usually not accurate.
Because of the severe weather late into the breeding season this year, we are seeing many late hatchings. So, this late nesting should not be of major concern and the eggs should not be disturbed.
The swans will leave the nest once the probability of viability ceases. Nature will not allow oversitting on eggs that have no chance of hatching due to the safety of the parents (i.e., predators).
Yes, female swans do reach an age when they no longer produce eggs. Birds are programmed with the exact number of eggs produced throughout their lifetime as well as the number they produce each breeding season.
Additionally, the younger the female swan is when she first produces eggs, the sooner she stops producing eggs later in life. Conversely, the older the swan is when first producing eggs, the later in life she will produce. This is all about hormones. The Regal Swan