I agree that there were no plot holes. (Though recall when the book was new posters on here felt there was too.)
The book is mostly written from Maud's prospective (though not actually in the first person). So if she doesn't recall, you don't know either. The frustration of the 'plot holes' will be the frustration of not remembering.
They may have brought her to the hospital before. We saw her walk out the hospital (just having seen Elizabeth )and tell her daughter Elizabeth was missing. Though I think it was clear enough the son had said no to her visiting.
The bandstand was the last time she saw her sister. She mentioned there was fish and chip and raced her home. Suki told her she couldn't run in the shoes so would follow. (No idea why she went home though - again Maud would never have know this anyway - met him on the way and got pulled home?)
Also why she hadn't worked it out. Well she didnt met Elizabeth till they were both elderly women. Hadn't been in her house or dug the garden and found her compact lid before she was starting to show signs of dementia. You get the idea she suspected Frank (and the lodger) but had nothing to go on - giving the location. It was finding the compact lid that raised the memory of the conversation about the buying the house. Then her living past memories as the present made her recall the 'mad woman' and well as the bird-thing. She repeatedly said she didnt know why she hadn't put it together (the 'mad woman's' comments about birds flying around).
Do think the mad woman's comments were reflective of how no body really listen (or was able to understand) when Maud was talking about Elizabeth being missing was her thinking about Suki being missing.
Both she was the mad woman had something important to communicate but neither could the dementia confused things.