Ariam went from a character I didn't care about to having one of the most powerful deaths in Trek history. I know she wasn't fully developed but Jadzia Dax was and her death was crap so that is not always a factor in a great emotional death.
Definitely agreed. I was one of the people who said they should have done the early scenes of Airiam interacting with the crew and reviewing her memories in previous episodes to develop her character over time. But if it had to be done in one episode this was extremely well done. I didn't care about this character before, but I really felt for her when even she herself was asking Burnham to open the airlock. Job well done by the actors, writer and director.
I was surprised about Spock giving advice to Stamets. How does he know anything about Culber? I guess his sister filled him in.