Locust Swarm

Is there any way to prevent a unit with the Demon Hunter model to go to it's Metamorphosis form when it uses a Locust Swarm-based spell?

If changing the "required animation" field for the spell to "Spell Throw" doesn't work, then no, there isn't.

A possible work-around is to give the Demon Hunter a summon spell, which summons a dummy invisible unit. Make a trigger for this dummy unit to cast your version of Locust Swarm the moment it appears on the map, and to follow the summoning unit (the DH).