I put down a load of Aftercut 4 in 1 fertiliser/’selective’ weed killer yesterday, as the lawn is full of weeds and not very green.
There are still some patchy bits which I want to patch with seed, but do I need to wait a while for the Aftercut to be absorbed before seeding?

I want to seed it today before the rain comes, but I’m concerned the Aftercut will be too harsh for the little baby seedlings…

yes….you must leave it for over a week following rain at least. Meanwhile rake out the moss as it blackens.