Personally I find MWI advocates to be shockingly indifferent to the details of how worlds split. If the notion of world is to be taken seriously, it ought to be a mathematically exact notion.
This might be nice, but we have to deal with what's actually the case. Wave packets simply don't divide into two at one exact instant. And if "it all adds up to normality" its not clear what use there is in introducing an arbitrary definition that allows you to say that a wave function represents one world at time t and two worlds at time t+epsilon. Whatever aspects of the wave function I care about, they only change by an order-epsilon quantity during this time interval. We could introduce mathematical function that takes in a wave function and outputs a discrete integer we call "number of worlds," but I wouldn't care very much about the output of this function. Even if I accepted that the "number of worlds" had executed a discrete jump from one to two, the worlds haven't diverged in any aspect by more than an order-epsilon difference.
Maybe we should call it the "Many-Blob Interpretation." That cries out for much less mathematical exactness.
And so whole decades can pass without physicists being forced to confront the question of what the state of the unobserved electron is, or of exactly when it is that one world becomes two.
Both Copenhagen and MWI answer that the "the state of an unobserved electron" is given by its wave function. Classical intuitions might suggest that an unobserved electron ought to have a definite, if unknown, position, but that's a failure of classical intuitions, not Copenhagen or MWI.
we have to deal with what's actually the case
Making excuses for an incomplete theory is not my idea of how to deal with reality. You can't just assert that a theory adds up to normality, you have to show that it does. And saying that you don't care about the details has no bearing on the logical need for such details to exist in a complete theory.
Both Copenhagen and MWI answer that the "the state of an unobserved electron" is given by its wave function.
In the original version of Copenhagen, the wavefunction is the state of the observer's ...
If you believe the MWI [1] you should care about the future a lot more than the present. Imagine you're considering whether to take a break and eat some chocolate in an hour or in two. You'll get similar enjoyment out of both choices, so you might think it doesn't matter. But if every quantum event between one and two hours from now will branch the universe, and there are lots of such events, in two hours there would be hugely many more yous to experience your chocolate break than in only one hour. The MWI implies we should be willing to make substantial sacrifices in terms of current happiness for the benefit of our future selves. In other words, your preference for investing probably isn't strong enough.
In trying to apply this to altruism you do need to be careful. Some charities are more like spending, in that their benefits are mostly in the present, while others are like investing. If I donate to the Against Malaria Foundation to distribute mosquito nets, the main benefits are preventing current or near-future people from dying. There are probably some long term effects, like a stronger economy when you have fewer people sick, but they're not the goal or the main effect. On the other hand the Future of Humanity Institute, a charity trying to prevent existential risk, is much more like an investment in that nearly all its benefit (which is really hard to predict or quantify) goes to future people. Metacharities promoting effective altruism, like 80,000 hours, Giving What We Can, and GiveWell, are another sort of investment-like charity, influencing people's future giving. And then there's the option of straight up monetary investing now and donating later.
If you accept the MWI you should be evaluating your altruistic options primarily on their future effects, with more emphasis on farther-future ones.
I also posted this on my blog
[1] Which I still don't know enough about to have an opinion on the truth of.