The timing that you've laid out should not be a problem as long as the Epsom Salts are spaced 2 hrs apart, 8:30, 10:30 and 12:30 for the Grapefruit/Olive oil mix. I have found that it's important to have the fresh squeezed grapefruit juice, so I would head out to that Publix if I were you.
I found that you do get thirsty during the flush so just keep some water handy to sip, but don't go crazy with the water or you'll only dilute the effect.
Since you'll be 2-1/2 hours late on your schedule, your first morning Epsom Salts will be at 8:30 AM or thereabout.
In prepping, if you haven't done so already, I find that it's better to have the lowfat breakfast on the morning before you start and just a snack for lunch, or no lunch at all.