Wait

so if I understand well what's going on:
Shouko-san travelled to the past to save Sakuta-kun from the Christmas Eve accident,
but failed which lead to Mai-san sacrifice to save Shouko,
Shouko-san got married with Sakuta (I guess

)...
and now Sakuta-kun want to travel to the past to save Mai-san
which can cause the current "future" to collapse
But, apparently
this is all just a Shouku-san dream... 
or ...
Damn 
I think I melted some braincell while writing this.