Welsh Harlequin laying questions!

[blankchicken]

So I'm pretty new to raising ducks, we rescued 2 Welsh Harlequins, a female and male. I'm not sure how old they were when we got them, but from looking at pictures and their coloring I think they were close to 4 month old. Now they are around what I think is 6-7 months old. My drake has been chasing one of the chickens around a lot lately, I know that ducks can be sexually aggressive. The chicken laid her first egg yesterday (so maybe he could tell she was fertile or something?), then yesterday my husband noticed the ducks were trying to mate. This morning I found what I think is a duck egg! It was on the floor of the coop, and near what looked like a hole someone was making (but not actually in the hole). It's also much bigger than any of the chicken eggs I have gotten. She wasn't sitting on it when I went in the coop this morning. Yesterday there was a tiny brown egg on the floor of the coop, but since it was so small (close to the size of a ping pong ball) I assumed it was one of the chickens who just didn't get the nesting box memo...could it have been a duck egg and just been small because it was a first egg?

So my questions are, is it a fertile egg since she wasn't sitting on it? I know she needs more than one egg to sit on, so how long will it take her to lay all of her eggs for her to sit on? Also if she lays more eggs and they are fertile is it too late in the year to have ducklings?
Thanks for any help!