The lack of subtlety could be down to the fact that this expansion is supposed to be a huge reveal of the true nature of various primary figures in the story.
Disagree, I felt it portrayed them wonderfully. Maybe you're forgetting they were on The First for years before the WoL arrived? People change as time passes. Y'shtola has become more open with her feelings which I felt was a good change. It makes her more interesting to me. She's still the deep thinker she always has been, but now we see how she really feels more clearly instead of getting mostly only a scholarly point of view.
As for the twins they acted pretty much the way I expected them to. I knew Alisae would try to be strong but would crack a little. Alphinaud is better at keeping his composure but he has always been more open about his feelings than his sister is. The motherly scene with Dulia-Chai is something I felt was done so well.
Part of this is everyone knows the WoL can travel freely between The Source and The First. It's not a true final goodbye because everyone can still keep in contact through them. Also Y'shotla explicitly said she intends to find a way that she can come back. She certainly does not think it's goodbye forever, but until she figures out a way so that she can travel with ease like the WoL does. As for Thancred I feel that he is trying to protect himself by not getting too attached to Ryne, even when he says goodbye. He has complicated feelings about her due to Minfilia.
And well Urianger mostly hung around pixies and I don't think I need to explain why there was no emotional goodbye scene for that. The pixies live in the present too much for an emotional goodbye and my impression is Urianger never got close to any of them, he merely learned how to work with them. Also a pixie spoke to Tataru in her dreams and while it was likely Titania who did that, who is to say the other pixies cannot do this as well? We know so little about the nature and the strength of their powers.
I did feel the scene with Lyna was rushed but that was due to much more pressing matters like...saving the world![]()
It was because Elidibus was panicking. He knew he was the final ascian figure who could lead his people in their quest, and I think he truly feared the WoL. Elidbus knew he was a primal and he knew the WoL has a long history of besting them. While he was far more powerful than the average primal, he also knew that the WoL vanquished an unsundered ascian so it was obvious that not even that aspect of himself kept him safe. He stopped being a cool-headed calculator as soon as he entered the scene after Emet-Selch's demise. He tried to overwhelm the Scions with sheer numbers and even appearing before mortal races, which was very out of character for him. I interpreted this as him acting out of fear and desperation.
To me this just makes it even more clear why Elidibus panicked. I seriously doubt all the ascians were perfectly dutiful and he would have known it. Some likely would have voiced their weariness of their quest, and it's clear the Asahi ascian has had his fill of it for a long time. We have a new thing to fear; an ascian who is trying to have fun at the expense of the mortal races.
OP I'm sad to see that you did not enjoy the story as a whole. While I disagree with you I'm not going to tell you that your opinion is wrong. Not everyone likes the same thing and that's okay.