I'd be happy to try bikes out, but they might disrupt the overall theme of the list too much like LeCacty said. The very first rogue trader death guard 40k list had bikers in it, so it's within reason in my opinion. There was only one 40k list that didn't have Nurgle bikers from memory. I was planning to run black legion to use my bikers, but this works too
On Blight Drones: I like the proposed 2+ to activate. I think their stats as they are are good, and they are probably priced about right for the 2+ activation. Having them as Elites keeps them from being spammed too easily.
On Grave Wardens: I like them as they are, but hadn't considered the similarities to the T-Sons. I'd be happy to try them out.
On Defilers: when I run a pack it is 3xAA and 1xP Hulk. I've never fielded the Defiler itself, so wouldn't miss it. But I see Atensions point that it is a good speedy formation, especially valuable if you don't have any blight drones.
Beasts of Nurgle: happy with the reduced FF value.
Plague Towers: I'd rather see them in the war engine section than the elites, so they are competing with other big, disrupting, hard to kill things. Seems more internally balanced to me.
But good work overall IJW! Keep it up