Senna and cascara sagrada are more damaging to the intestines...the only problem with Epsom Salts ...or anything else that gives you prolonged diarrhea...is dehydration and possible electrolyte imbalance. Both of these are easy to prevent or correct...just make sure that you are always well-hydrated and use an easily absorbed mineral supplement that includes trace minerals.
BTW...I think the main reason you were nauseous with the last flush is because you ate too late prior to drinking the oil...you need to eat fat-free the day of the flush and fast for a good 6-8 hours before you drink the oil mixture. That much oil on top of a recent meal would make anyone wretch!