"It’s odd because she waited so long to do it. 45 minutes later.
If she was concerned about Nicola why didn’t she call someone immediately or very soon after? And if she wasn’t concerned why call anyone at all?"
The context of the call hasn't been released. She could've been ringing her DIL for anything and then mentioned it in passing.
Eg. Just had my appointment. Doctor says bloods are normal. I can pick Daisy up from school this afternoon. Guess what? I found that dog running loose again, the brown spaniel? Tied it up, it should be on a bloody lead. It was a bit weird because there was a phone on the floor with a photo of that woman from Daisy's school, 2 young girls?"........DIL that's Nicola and Willow, I better ring school and they can check she's ok.
Same with the "play date". This was a text sent to a friend who's mortgage she had arranged just the night before. The text could've been something like,
"Hi, thanks for sorting my mortgage. What a relief. Anyway, do you fancy getting together with the kids at half term, we could go bowling, my treat".....Nicola "Yes, sounds good"
By the same token the woman could've rang her DIL worried because she'd had to leave in a hurry but something felt off and Nicola could've instigated the play date conversation.
My point is there are many explanations and nothing points to one obvious outcome at this point.
Initially it seemed obvious she was in the water. Now it seems much less likely.