Sc. 1, lines 89–114: how were the elements of the friar’s plan foreshadowed when the friar first appeared in the play? why is the timing of the friar’s plan critical?

The Friar's plan is foreshadowed from his first appearance in the play because the first lines the audience hears him say are all about the specific things he knows about plants. He knows exactly what each can do and it's clear that some of them are more dangerous than others when made into potions and mixtures. 

The timing of the Friar's plan is critical because Juliet will appear dead for an exact amount of time. If she wakes up too early it will ruin the plan because then she will have to marry Paris, but if she wakes up too late she will be all alone in a tomb without a way to get out. The plan also relies on communication reaching Romeo in time so he knows she is not really dead.