OH! Those poor things :-(
I am very sorry your ducks died.
Breeding your Welsh Harlequin with the Blue Swedish sounds like a good idea
ducks are very sociable and she could get sick from being too lonely.
as for when they stat to lay,
I just did a little research and the most common answer to this is about 6-8 months,
though a few people said there duck didn't start to lay until about 9-11 months.
I would suggest you wait until she has been laying for a least a month before you breed her,
just so you get good strong eggs.