You have been very busy my friend! I like the fact that you are attacking this from all sides. Sending very good vibes your way and hoping for the best.
Keep us posted!
I AGREE with chickenlittleton. If the DE is FOODGRADE (please make sure) it will not cause problems. I thought the bird seed was suspect but you did say that it will sprout if left on the ground. Good. It's untreated.
I believe the mulch is the problem. chickenlittleton is correct. Bagged...