I’m afraid I haven’t got any suggestions to offer to help with downy mildew. It sounds very frustrating and dramatically ruinous in the speed with which it takes over. Great that the netting helps, though I’m surprised and puzzled by that.
But I also commiserate over the need to use netting. I have to cover all brassicas and alliums too, but for a different reason - rabbits. And they’re great masters of finding any loose fixing in the netting and getting in for a good munch anyway.
I hope someone can suggest a more resistant leek variety but a quickish google isn’t very encouraging on that score. ☹️