Yeah, the cutting early strikes me as a preventable loss. If his temperatures are a degree cooler than he thinks they are, his babies could be as much as a week farther behind than he thinks. And all he had to do was wait another 7-10 days and he'd have had two bees.
Mold happens, honestly. Maybe it was a roll-out.
Slugs, too. That's trickier: that might be pairing practices, but it could also just be those females and this year. What is his pairing procedure?
What happened with the last clutch from the normal?