Let's not be so quick to criticize this sheriff and his team. They've had one of the rare successes in this type of case, and most likely it's because they did everything right. Keep in mind that they did indeed know SB's background -- she already had a record in this county due to her (and JB's) extradition to Beaufort County, SC in 2000. They knew she (and I think JB too) had been formally charged in connection with the disappearance and presumed murder of 3 year old Paul Baker. They knew she had been convicted of severely beating a young girl in her care, Paul's sister. In other words, they had good reason to believe this is someone who assaults and KILLS children. People like this can decide to kill without hesitation, the second they think it's in their own best interests.
LE had Tina the whole time -- not sure if she was technically "in custody", but she would have been quickly if she'd tried to leave (they may have had her in some sort of "protective custody" due to her mental disability). They were keeping her away from SB/JB and probably getting a lot of information from her. They may well have known early on that Tina had given Shannon to SB and that the plan was for SB to take care of Shannon. Their number one priority had to be not triggering anything that would cause SB to kill Shannon, before they dould plant themselves physically between SB/JB and the presumed location of the Shannon (which dogs may have identified as SB/JB's house very early on).
What they needed NOT to do was head over to the house with SB or JB in it, with SB (and I think JB) knowing exactly where in the house Shannon was, while all police knew was that they were pretty sure Shannon was somewhere in the house. If SB saw police coming, and Shannon was starting to fuss enough to be heard from the box under the bed, SB could very well have decided to kill Shannon on the spot. A quick opening of the box to strap duct tape tightly around Shannon's mouth and nose and wrists, close up the box -- less than a minute, when she's planned for this possibility -- then invite the police on in with confidence that there would be no baby noises. I'm really, really glad LE didn't go that route.
We don't know all the details of what LE knew and when, but it's unlikely they did anything really stupid. They were no doubt hampered a bit by the need for either permission from SB/JB for a search, or a search warrant, and I don't know why there was several hours delay (apparently) from the time LE got SB and JB out of the house, to when they went into the house, but I'm perfectly willing to assume at this point that they knew a lot about the details fo the situation, and were basing their decisions on what they knew.
I also don't recall Shannon having a "condition" -- only hearing horrible remarks that she looked like one of the 7 dwarfs (that still makes me mad). Does anyone have a link that confirms Shannon has a "condition".