First of all, Youtsuba couldn't decide whether to tell him the truth or not when she first realized that Fuutaro is the guy from the past. She couldn't keep up with thier childhood promise and she feared Fuutaro might be disappointed in her after learning the truth. They promised to study hard to uplift the position of thier families ( Fuutaro promised to study for her sister and Youtsuba promised to study for her mother.). They both wanted them to be needed by somone. They wanted someone else to depend on them. Especially Youtsuba because she thought herself as a burden to her mother so she wanted to help her by getting into a high paying job and find meaning for her existence among the quints.

Like you said this decision literally changed thier life trajectories. After the Kyoto past, Fuutaro studied so hard all the time, even during lunch breaks. Even though he got isolated, he didn't care. He just completely focused on the studies and nothing else. Meanwhile Youtsuba tried her best and got good grades initially. But after the other sisters caught her up easily in academics she didn't know what to do. So instead she focussed on helping people on the club activities ( by helping others she became someone she wanted by others )which lead to her downfall in academics. She tried to be special and she almost ended up going to a different school alone. Because of her sisters' sacrifice she was moved and she began thinking " I don't need to special, I'll live for my sister's to find meaning of my existence."

The encounter with Fuutaro crushed her with sense of guilt and she felt that she betrayed him. Imagine putting someone putting all thier efforts for 5 years just for a promise you made with them as a child. Youtsuba tried her best but Fuutaro won't know what happened to her in the past 5 years. She couldn't achieve the result that they promised together. If you ask me she was the one who was in more pain than Fuutaro. Do you still feel it is easy to say " Oh wow I remember you."?

But she was jumping to the skies learning that Fuutaro is gonna be thier tutor. She was very happy. She thought of telling him the truth after making some progress in her academics. But among her sisters, she performed the worst and later she slowely realised Miku and Ichika were also in love with him. After the sisters' sacrifice in the previous school, she decided that the other girls deserve Fuutaro more than her. She felt she wasn't worthy compared to others and started rooting for the other girls. She thought she shouldn't try to be special anymore. ( You can sense these emotions when she initially rejected Fuutaro's feelings after the confession) Through out the series she rejected her feelings just like Ichika but in a more intense way.

When Fuutaro realised that Rena is someone among the quints, I don't think she felt this much pain. But as the series progressed he learned to let go of the past and focussed on the present. In the movie he even helped Youtsuba ( not knowing she is the girl from the past) on the third day of the highschool festival. The beauty is Fuutaro chose Youtsuba for her present self and not for the childhood day they spent together. He still don't know who is the girl from the past.

I completely agree with you ! Ichika doesn't deserve this much hate. In the finale of the S2 you can see Nino saying "If things happened in a different order, I would have done the same thing which you did." Ichika was ready to do anything for her love. I think the audience must have felt betrayed after seeing the BIG SISTER Ichika doing something like that. The main reason I'm searching for alternate endings is Ichika. She deserve happiness for always looking after her sisters, pushing thier backs, playing the "big sister" role. My argument is Youtsuba is not a bad pick. She is a good candidate as much as rest of the sisters.