Overfilling by about 400ml each time I changed my fluid, do you think that caused my seals to fail?

Not if the transmission vent is not plugged.

The only thing that causes a seal to leak is a bad seal, damage to the shaft that the seal is sealing against or the shaft is so loose in the hole in the case that the shaft can move to the point that the seal is no longer touching the shaft around it's full circumference.