One drawback with clear and precise rules, is they ruin a lot of the story by sacrificing the fun descriptive text for clear rulings on spell interactions, and remove any rule which cannot be written in such a way as to allow for clear and concise rules.
I'll agree Magic Missile can be interpreted in a number of ways, however the errata still fails to be clear and because of that I would use the obvious answer of "it's up to the DM." We regularly get into the position where DMs don't like this, but DND 5th edition is built around letting DMs make rulings, while previous editions are for DMs who need precise, clear, easy to understand rules.
I, personally, would allow each dart to apply the bonus purely because as a 1st level slot, 3d4+18 damage to one creature is neither overpowered, nor worth quibbling over when the level they obtain this is only 1 below adding a third dice to cantrips for a 3d10 output (sure the lack of attack or save can be important, but even then... still not worth quibbling over when Conjure Animals lets you form 8 Giant Owls who deal 2d6+1 damage each in a flyby attack, allow each of them to flank with a single person in some kind of devilish aerobatics display). Power levels of spells is very important to consider when you make rulings and I love the fact 5th edition allows for interpretations now, that I would've hated 15 years ago during my era of "by the book" 3rd edition.
I'll agree Magic Missile can be interpreted in a number of ways, however the errata still fails to be clear and because of that I would use the obvious answer of "it's up to the DM." We regularly get into the position where DMs don't like this, but DND 5th edition is built around letting DMs make rulings, while previous editions are for DMs who need precise, clear, easy to understand rules.
I, personally, would allow each dart to apply the bonus purely because as a 1st level slot, 3d4+18 damage to one creature is neither overpowered, nor worth quibbling over when the level they obtain this is only 1 below adding a third dice to cantrips for a 3d10 output (sure the lack of attack or save can be important, but even then... still not worth quibbling over when Conjure Animals lets you form 8 Giant Owls who deal 2d6+1 damage each in a flyby attack, allow each of them to flank with a single person in some kind of devilish aerobatics display). Power levels of spells is very important to consider when you make rulings and I love the fact 5th edition allows for interpretations now, that I would've hated 15 years ago during my era of "by the book" 3rd edition.