OK, I thnk I've found something that might help explain the estu/estus issue with this exercise. In a different comment stream, but on this same exercise, one of the commentators said that "estu" here would be like a (polite) command.
So I tried a version of English that carries that connotation, and IT WORKED! (It was accepted.)
is to / are to in English can carry the sense of command: The file is to be ready tomorrow (ACCEPTED)
Other examples in English (I'm making these up): Students are to hand in their papers on Monday. Lecturers are to arrive ahead of time. Parents are not to leave children unattended.
BUT: Drivers must not use cell phones while driving. (This is too serious a prohibition to muddle around "politely" with the is to / are to construction.)
It would be interesting to see how some of the more expert Esperantists aboard would translate these.
I think the confusion mainly arose here because of a (perhaps too-liberal) use of "should" for any and all situations where it's a possible translation. At this level we're coming face-to-face with finer shades of meaning than in the earlier levels. Is to / are to allows for some of the shading that Esperanto seems to imply with estu / estus.
No. The "must" is just the -u ending.
"estu" is more like "shall be".
Vi estu pli singardema: Be more careful. You should be more careful.
Tiu kuko ne estas bona. Refaru ĝin, kaj la venonta kuko estu pli dolĉa. This cake is not good. Re-make it, and the next one shall be/should be sweeter.
So it's not "should be ready tomorrow" in the sense of "I guess it'll be ready tomorrow" but in the sense of "it had better be ready tomorrow, or else".
Am I the only one that feels estu is confusing? Saying, "La dosiero devus/devos esti preta morgaŭ" or "La dosiero devus/devos preti morgaŭ" seems so much more straightforward. Now if I were talking to a person about their readiness, I could see saying, "Estu preta morgaŭ" (Be ready tomorrow).
I just got a different version of this question and it does accept "... devus esti ...".