I don't recall any mention of locking down the phone (I'm pretty sure a phone can be answered from a remote location anyway - I wonder if that technology was available back then).
Impractical, tricky and unnecessary. It wasn't going to happen, there was no need and probably couldn't like you mentioned.
I also don't buy that LE would not usually (and immediately) be very interested in any forensic evidence that would give them information about the identity of any kidnappers, point of entry etc., or the condition of the victim.
They would be interested. The victim would be assumed to be elsewhere....why else leave that ransom note? If she was dead just leave.
I just can't see why LE would delay a thorough visual search at least - that could have been done even if the plan had been to keep the Ramseys and their friends close to observe their demeanors and interactions.
They didn't delay. They just didn't do it.