I have watered with H2O2 and lost seedings from daming off and mold. I have looked for a water soluable product that protects against desease like damping off. Nothing is available in the local stores so I would have to order something. My solution is to move everything outdoors where I have not had any problems before. I know it is possible to have issues anywere. We are not getting regular rain like you got on your plants. The problem is our weather with temps near 100. I am used to working out in the heat so that is how I am starting seedlings for now.
We am planting lots of miniature and patterned crosses. We have about 200 seeds planted now but after this weekend it should be about 2000.