By the time Jean gets back, it'll be/it will have been too late

[svetlana14]

In the sentence "By the time Jean gets back, it'll be/it will have been too late", the right choice is "it'll be". I understand the subtle difference as follows: Under the first “scenario”, there is a probability that “something bad“ is in the process of happening while the second option suggests that the bad thing has already happened and there is no chance to prevent this from happening. The logic dictates that two options are vital and it is the viewer’s perspective of how to look at those events as there is no additional context is given in the test. However, the right answer is just limited to “two”. Please explain to me why you think the choice is limited. Thank you.

 

[emsr2d2]

I'm not entirely sure what's confusing you. "By the time Jean gets back" puts her return in the future. Logically, the rest of the sentence needs to refer to the future too. Only "it will be too late" fits.
It doesn't matter if the thing that is the reason that it'll be too late (we aren't told the reason) is happening right now, has already happened, or hasn't yet happened.

"By the time Jean gets back, it will have been too late" is not grammatical.