1) There's also your body, in the arms and legs and torso sense, which is you, but it's not your brain. There's also your non conscious part (whether you want to include a Freud style subconscious, or autonomous functioning parts that reach a decision way before it has registered as a conscious though) which is your brain, but not exactly "you" as you think of you. Plus all kind of body-produced chemicals and hormones that influence you, despite not being "your brain" - they're still you, since you can't ever separate the two (the thoughts and behavior you have on them versus what you'd have without them. There is no second you to have a test, do a diff, and be able to tell exactly which behavior/thoughts come e.g. from an excess of those chemicals (e.g. due to exposure or diet or some condition in your body) and which from your regular brain without such influence.

2) "Nobody has ever experienced a crystal ball like this" Irellevant, as many do act at many points in life as such beliefs are 100% certainty (a plotting killer believing they will get away with it, for an extreme example). Doesn't matter whether they have a "crystal ball" guarantee of outcome X: their thinking making them see the future as having the same outcome X is enough.

As for (3) I sorta agree.

"MacAskill and others are concerned about what the discount rate should be because it is important for the decisions we make about the world—how much should we care about people who have not been born yet? One might be tempted to think that science could resolve this issue, but alas, it cannot."

Of course it cannot. It's an ethical decision, which rests on personal ethics (not totally unlike personal taste), which are influenced by cultural (society's) ethics.

If we want to think "rationally" about it, it would be based on "what kind of outcome we want for those future persons and how we can best achieve it". But even the choice of outcome would rest on ethics and cultural preferences - or something also non-rational like evolutionary urges.