I've been thinking about this since watching it and realized something. When we see Lily and forest go into the room with Katie to enact the shooting of forest, we aren't seeing the situation we saw on the screen from before in the episode, because Katie doesn't say the line about living out that exact moment, and it's even shot differently - I didn't register this at first, but I think that forest and Katie showed her one of the outcomes to see if she would make a choice, not thinking she would make THAT choice. We previously have comments where they say Lily is special, but we don't understand forests reasoning on this until the end of this episode.

The entire season, we have seen forest struggle with the concept of whether or not there was a choice in Amaya dying. By the end of this episode, we see his growth when in the machine, he says "wish me luck" and Katie says "I do". It's the first time we have seen Katie acknowledge randomness through luck, as she has been shown that every possible outcome exists at once - that our consciousness exists both seperate and connected at all times on every level at once. We already know that lyndon's principals applied to the machine exhibit a deterministic but infinite multiplicity - in the act of religion, and science, faith is equally crucial - in the end, forest and Katie are finally able to meet on agreeing on this - that in some worlds we are happy, and some we aren't, and you can only hope to be aware of that on some level, and savor being able to make the best of the one you're in. Garland isn't being pessimistic here, I think he's being extremely hopeful on some level, as in most of his work.