I'm realizing we're probably both confused by the word "belief."

Newtonian physics has been proven wrong, but it's still a perfectly reasonable system to use for, say, landing a man on the moon. I thus have the belief "Newtonian physics is inaccurate under certain extreme conditions" and "Newtonian physics is an accurate-enough model for a specific domain of problems, which includes basically every practical problem I am likely to run in to in my daily life".

Given that both MWI and Copenhagen are accurate models of the data, I would think that it is accurate to have the belief "both of these are accurate models of the data; I can use either one and will get equally accurate predictions". One could then have additional beliefs, such as "I think MWI is closer to Universal Truth" and "I think Copenhagen does a better job of telling a story that is compatible with my conscious narrative".

It seems important to me not to get these multiple beliefs confused in to a single simplified belief such as "MWI is true" or "Copenhagen is true" or even "Newtonian physics is true".

So, when I said "I'd assume THE proper belief is...", what I really should have said was probably "In addition to the belief you just stated, you can still ALSO believe that they're both accurate descriptions of experimental evidence."

tl;dr: Good point on Copenhagen vs MWI, but don't let yourself forget that both of them are accurate models of our experimental evidence so far. If there's ever a test between the two, you should probably only be ~50% confident in either one turning out true :)