Also wondering the same thing, and whether I missed something! I went back for multiple playthroughs of Dallas's route, trying to get By The Pier & In the Dark, not sure what I was doing wrong. Even with all the dates/other scenes unlocked, and with me delaying the time until the AMPs by several months to make sure it was after July 2 (per the walkthrough), I just wasn't sure how to get to these two scenes. Tried hanging out with Dallas a whole bunch, but that didn't seem to be it.
That said, I used the find-memo cheat because I just had to see these scenes, and I completely agree with the OP! These scenes are absolutely necessary to the plot and I'm glad I had a chance to read through them. It doesn't really make sense otherwise why Dallas leaves at the end (and it's doubly weird because we see part of the "By The Pier" scene in Beau's route). It also flows really beautifully in the ending-- before getting these two scenes, I found the ending get-together abrupt and wished we'd gotten more romantic scenes before.