Simple RAW for 5e V: The Disadvantage Strikes Back

[Hairfish]

Q417

After you break Polymorph with the first dart of Magic Missle, do the other 2 still hit? Does it count as the same creature?

A417

I'll use the example of a dwarf that has been Polymorphed into a bear in the hopes of making the answer easier to understand.

Magic Missile's spell description explicitly states that all the darts strike their target(s) simultaneously, so there is no "first dart". The darts all hit the bear, damage is rolled, any damage beyond the amount needed to reduce the bear to 0 hp carries over to the dwarf.

I'm not entirely sure what you're asking with the second question.

If you're asking if the morphed dwarf has their bear form reduced to 0 hp by the first shot of an Eldritch Blast counts as the dwarf or the bear for the second shot, then they would be the dwarf.

If you're asking if the second shot would miss the dwarf entirely because it's a different "creature" than the bear, then it would not miss the dwarf. "Creature" in this case is a specific game term and both the bear and the dwarf are the same creature.