This is my personal opinion... I would look to simplify, especially with a plugin.
Perhaps you could later upgrade to a more complicated setup, but if it were my plugin, I would try to force only one mailing list at at time.

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you could use checkboxes (can't remember was it aweber that does that when you manage subscriptions.
then you could accumulate errors in an array and then show them at once.

you could leave that for the version 1.1 if you want.
release as soon as possible