Yes, I see this as a kind of passive direct damage spell, so my intention is that Mirror and Jade would deflect it.
Fractal could be a problem though. So if Fractal is the biggest concern, I see three ways to tweak it suggested (feel free to add more):
1. Make it 1/1 so that Fire/Frost Shields have an effect.
2. Cap the amount of damage per swarm.
3. Increase the cost per swarm, slowing down a Fractal rush.
Of the three I would prefer number 3, which would leave the question of just how high the quanta cost needs to be so that Fractal is still a viable option, but not a broken one (I know, I know, Fractal and "not broken" are hard to put together). Would reducing the cost of Onerous Swarm to 2
and increasing that of the Wretched Swarm to 4
be sufficient you think? Perhaps 5
?