Taking it from an innocent point of view, she may have assumed that she checked on her at 10:30pm if it was her routine to do so before she went to bed (it was ours when our daughter was little). The sickness to me is irrelevant, unless it was something serious. That's what baby monitors are for if it's a cold. Now, I know it's said that no one can confirm there was a baby monitor out there, but we are relying on the words of SB for that info, someone who hasn't talked at all about details about that night.
I think saying initially that she last saw her at 10:30pm was a CYA moment whether she's guilty or innocent. If she's innocent, she may have thought that she should tell them 10:30pm, whether she truly remembered doing that or not so that it doesn't look like neglect (perhaps why she always thought she was going to be arrested).
I struggle with believing how any human being, much less DB can sit on her fanny for 4 plus hours in relatively one spot, while drinking and not going inside once to either check on the kids or even go to the bathroom.