See who you are, where from, what of
Yeah, you're right, the type switching does seem a little awkward. I don't really know a good way to remedy that either. Also, yeah, it should only affect adjacent Pokemon in triple battles.Miasma Pulse - Sure.
Blight Dart - It could be physical, as the dart is fired from the user's body with its physical force. Otherwise, it's alright.
1) The effect is rather unique, and it's fine to badly poison more than one Pokémon at a time. However, it shouldn't affect non-adjacent Pokémon for the same reason that Sludge Wave doesn't.
2) The type-switching to Normal seems a bit odd, as does the automatic failure if the user is not a Poison-type. The two are just for balance issues to prevent the Pokémon using Corruption from dominating the field in case the opponent switches out and you try to use Corruption on every single Pokémon they have. But if so many limitations have to be put on a move just to keep it balanced, it probably does too much.
3) The problem then becomes the fact that if the type-switching and automatic failure drawbacks were gotten rid of, this move would be strictly better than Toxic. An easy fix would be that the less PP it has, the more chances the move has of not connecting. For example, at 100% PP (5/5 or 8/8 when the player selects the move), it would have 80% accuracy, then it would decrease by 10% for every 12.5% of the maximum PP of the move that's used up, rounded down.
The flavour behind changing the user's type would be that it uses all the poison in its body for the attack, but that makes no sense, especially on Pokemon like Grimer that are essentially made of poison. Other than that, yes, it was just for balance purposes to prevent spamming it alongside a Poison or Steel typed partner with a phazing move.