So if Promising Future resolves and one or more players choose to banish Nocturne from the cards they looked at, what happens?

As you go around in turn order and each player looks at their top 5, they can choose to banish one or more Nocturnes before they banish a card with Promising Future. Any such Nocturnes will go on the chain as pending items before whatever that player chooses with Promising Future. They’ll remain pending through the whole process of resolving Promising Future, then at the appropriate point they’ll be fully placed on the chain and (because they’re units) will resolve immediately.