you need to work a good potting soil into the clay and loosen soil larger than plant root. I dont know how your plant drowned never had any problem with that and an outdoor grow. be careful with manure that you dont burn your plants. you will have to let it compost for a while.
i had the most beautiful plants last year and just about harvest time a four day rain came. i was waiting for a light frost could have been devastating. lost a lot probably affected the taste. i trimmed alot.
let your plants get a little root bound in the container that they are in. dont mess to much with seedlings. sometimes if they get to spindly you can just plant them deeper in same size pot.
last year it was time to harvest and about a three day rain hit. mine started to mold i separated the buds and cut all the fan leaves off. watched closely and trimmed all mold.