I've found grapefruit juice to be the gentlest of the citrus juices used in flushing, so I don't know that switching to lemon or a combo of lemon/orange/grapefruit would help you? :(
Mostly it seems to me that you simply need to prepare your digestion to deal with it all, and consider taking smaller doses of the oil/juice AND the Es too. The Clark doses are quite appropriate for a 200+ pound male, but not so great for smaller people! :)
I take just 1/2 cup of oil with 3/4 cups juice, and then 30 to 45 minutes later, if I feel okay, I take a second dose. That gives me a chance to back out if there's a lot of nausea. Nausea at that point is a signal that a LOT of toxins have been released and the body is trying to decide how to get them OUT of you the fastest. Purging/vomiting is often the very fastest way. Yikes!
People often only need one dose of Es, not two. I never needed two doses. And I prefer doing enemas rather than the morning dose of Es, much gentler to the body.
Be sure to drink ginger tea, and peppermint tea, during the day prior to flushing. Eat several cloves of smashed garlic too. That will prepare your digestion for dealing with all that oil.
After you take the first dose of oil, suck on hard peppermint candy or Altoids. that will take down the nausea! :)