I'll use italics to refer to direct discourse (literal quotations) and bold to refer to indirect discourse
(reports of quotations).
She told Rose, "Jack'll [Jack will] come tomorrow"
gives the direct discourse, that is a direct quote of the literal words of Rose's informant (the "she" of the sentence). The version
She told Rose that Jack would come tomorrow.
is the transposition into indirect discourse.
When you keep the indirect discourse in the question
When did she tell Rose that Jack would come?
you're asking about the time within the indirect discourse, that is, the time reported of Jack's arrival.
If you want to ask about the time of the telling, you ask
When did she tell Rose, "Jack will come tomorrow"?
keeping the direct discourse.