Toxins can be removed - funnily enough the Icelandic have ridiculous methods to remove toxins from poisonous sharks and make them edible. It takes months of process but it's also believable they'd try it on dolphin and seal trolls during the famine.
I know there's a certain degree of magic to the illness and beasts, though.
What we do know for a canon fact is that the blood kills off mosquitoes as they ingest it (which they will, so the chemical cues they act on must still be there). Dead grosslings getting reduced to skeletons might be due to sheer physics/chemistry and the body not being able to repair damages anymore, rather than something living decomposing it; remember that, depending on plot needs , grossling bodies can prove surprisingly frail ...
That's not really how physics/chemistry works. When the body stops functioning, without
any decomposers, it would dry up but just lie there. Changes wouldn't happen because there's no longer the catalyst of life to make them happen.
It might with the more corrosive kalma - but then some trolls mutate to survive the corrosion.
The reason dead animal flesh doesnt just sit around is because
something is eating it.
Rotting is due to bacteria or fungi. And often from bacteria/fungi already present in the creature hence why it starts so fast- although the air is also full of it. Even without any insects/birds, decomposition happens.