OP - I am sorry that this happened to your DS, it must have been so upsetting for you 
My toddler recently broke his arm at home. No-one saw him fall and I thought that he was unwell rather than hurt. He was very pale and a bit sweaty looking but seemed content to lie on the sofa. He only cried when we tried to sit him up.
It was 24 hours after he had fallen (roughly as we don't know exactly what time it happened) by the time I got him to hospital. I took him there still thinking he was ill - although by this time I thought he was very ill as he was drowsy as well
It turned out he had a broken elbow.
Because I didn't couldn't explain how it happened, I was questioned in detail by three different people. At the main hospital, I was taken into a separate room and really interrogated about what happened - for safe-guarding reasons.
The Dr in charge also went and looked up literature about the type of fracture my DS had. The type of fracture can show if it is likely to have been caused by an accidental or non-accidental injury.
DS's injury was obviously an accident but I felt terrible that I wasn't in the room when it happened & I didn't realise he was hurt. The questioning I got from the hospital was upsetting but I understood why they had to ask.
In this case, the nursery do need to answer questions. It sounds like they saw your DS fall but they didn't tell you what happened. They didn't notice that he was pale and irritable from then on - or if they did notice, they didn't tell you or do anything about it.
When parents are routinely questioned for safeguarding reasons about the possibility of a 'non accidental injury' surely nursery staff should be questioned in the same way, for the same reasons?