H202 will help with rot and pathogen control but not mold.
Same problem too wet. Also the warmer ambient summer temps make the mold much worse and spread faster.
No thats incorrect , cuttings draw water up thru the stem cut and dont use nutes until they have a bunch of roots under there.
The reason you have mold is not from poor sterilization its from the humidity being too high in the dome, try to avoid domes and the medium has to half dry between...
Its not P def. Purple stems usually mean its stressed.
Clones that young and starting out dont need much P so you can scrap that theory .
It takes time for the clones to take to the medium and start growing. Dont feed them yet and reduce watering.
Its very def in K. When plants are not taking up K they have no defence against disease and this is what happening to your plants.
I think its a ph/ feeding issue you need to fix asap. Maybe foliar feed with potash liquid.
Those sick ones will struggle to come back.
Stop watering they are very overwatered and stressed because you have them in a dome with high humidity. You dont need a dome with seedlings at all. Also now they have germed let the pellet dry more between waterings.
Not sure if these ones will pull through they are sick,stunted and yellow...
They look like dud seeds, twisted and stunted. Not much you can do just wait and see if they grow out of it. Put them back in the sun and stop watering until they grow again.