I think it is kind of inescapable that we need support for user merge for big mailing list imports, I have seen quite a few cases in old mbox imports where users switch emails a few years in.
I think merge is the most robust way of solving this, but we are going to need a few features first to unlock this, more than one email per account is a big one.
Agree that streamlining on-boarding is good, but account merge is the thing you really really need.