First of all.. ALL THOSE DEVELOPMENT DOWN THE DRAIN, NOT ONLY DID JUNG AH DIDN'T SEE NA KANG LIM SECRET, THERE'S DO HEE WHO STILL ISN'T AWARE OF HER FEELING, PLEASE TELL ME HE'S STILL GONNA DO THOSE STUFF
If bro changes his decisions this loop, he still needs to go on the date with Jung Ah!
Because if he doesn't and Jung Ah doesn't look at his phone and find out the truth she'll be manipulated to go yandere.
After all the only reason why she resisted the temptation of the sister's plan was because of the responsibility she felt she had out of being the only one that knows the truth.
Without that, Jung Ah is highly likely to go along with the plan out of desperation.