Of course it had to be spelled out - its an episode not a book where we can examine what everyone is thinking. If they had just suggested that Edith was a focal point and she must die or "bad things" would happen I'm sure we would have all been complaining.You can blame all that on the Trek staff. They stamped "SHE LET'S HITLER MAKE NUKES" on her, whereas Ellison never explicitly spelled out why she's a focal point, just that her death by way of an auto accident is a crux point. Spock merely suggests her way of thinking might spread and delay US entry into the war as a way of illustrating to Kirk the sorts of ways a small change might alter future history. The real themes of the story got whacked by Coon, Fontana and Roddenberry.
The hackneyed dialog the staff wrote to replace Ellison's didn't help, especially the dreadful Edith predicts the future scene. The casting probably didn't help.
Bad sex is awful. Some pizza is almost as awful.
Maybe it would have been better if Kirk was in denial and said to Spock he couldn't prove any of it and still insisted that McCoy must be stopped from killing her but realised in the end and did the right thing.
I'm still very happy with the episode and its one of Star Trek's best. Perhaps the future scene could have been toned down but just a bit of tweaking. I liked the romance except the short time period but there's been worse.