Is this a realistic response from a mother who is panicked after discovering her daughter missing? Could you even think about precise times? Wouldn't it be more likely to say you got home late?
Remember Damon's concern about the timeline???????? Maybe Brenda was thinking a lot about establishing it right off the bat.
Other oddments: Damon's early morning vacuuming (denied by him on the stand, but then again, he has a little "truth" problem), the ever-changing time-lines, the way Damon told Brenda not to get into the timeline at one of the press conferences--of course there's lots more, but these are some of the early warning signals that sharp police should have caught. Why didn't they?