With the poisoning theory, as with the murder theory, it is reliant on DC not being discovered. Most of us suspect that Simon was poisoned without discovery and her panicked actions started straight after DC were mentioned.
With the poisoning however, EP is far less of a suspect because she has after all gotten ill against a spectrum of illness. This is far more believable than 4 on their death beds and one person having diahorrea. Even if they hadn't found DC, she would have obviously been the prime suspect. This isn't necessarily true with poisoning, I don't think she was suspected about Simon's illness although I'll admit I'm not sure.
Without identification of the poison, there could have been a number of different possibilities, some innocent some not. There also wouldnt necessarily have been significant police involvement like there was with deaths. This is possibly naive, but not as naive as thinking 4/5 would die and you wouldn't instantly be the suspect.