Will overdue duck eggs hatch?

A month ago I received 22 fertile duck eggs purchased from ebay (12 Swedish and 10 Welsh Harlequins). I immediately placed them in an incubator with temperature 99.5 degrees and humidity 60%. I used an automatic egg turner until day 25. I had 3 hatch as scheduled on day 28. 1 hatched on day 29. I helped 2 hatch today (day 30) and they are all doing fine.

6 ducklings out of 22 fertile eggs seems like a very low hatch rate.

I also raise chickens, hens only. I know you can float test infertile eggs to check for freshness. Bad eggs will float, fresh eggs will sink. I have read online you can test overdue duck eggs by a float test and the results are reversed, viable fertile eggs float and bad, dead, or dud eggs sink. I candled all the unhatched eggs and some are dark some are clear. Some clear eggs floated and some clear eggs sank. Also some dark / full looking eggs floated and others sank.

What are the odds some of the remaing eggs may still hatch? I am very doubtful, but I will incubate them for a few more days to see what happens.