Most man hole covers (and gutters) in suburban areas are in plain sight of many neighbors. If Casey borrowed the shovel for an hour in the middle of the day, and neighbors were attentive enough to notice her pulling backwards into her family's driveway was out of the norm, them I highly doubt Casey would have gone unnoticed opening a man hole cover on her street and dropping a [large sack] down the hole. After all thier testimonies, sounds to me like one of the Anthony's neighbors would have noticed and inquired if Casey messed with a man hole cover.
Point two: have you ever lifted a man hole cover? Casey is a size 2/4, probably no more than 120lbs with twiggy arms. Even if she could get away with lifting the cover with a shovel in broad daylight, she'd still have to manipulate it off and back onto the hole. The covers are intentionally very heavy.
i dont know the area, but i agree it would be a tough one to get away with
but it could be done in less than 15 seconds
yes, they are heavy - but it can be done
pry it open with the shovel - use something like a back pack filled with a couple of PAVERS to put underneath the cover and throw the child in.