Oh boy, I don't envy you! I've never had an attack, but I hear that they are awful.
Epsom Salts in capsules may be less effective if they dissolve later in the digestive process -- at a minimum, the capsules delay absorbtion, so it may take more than 30 minutes to feel relief. I think that is why the flush recipe tells us to drink it dissolved in water.
What if you dissolved Epsom Salts in apple juice? Could you drink that? Others suggest a long hot Epsom Salt bath to help one feel better.
If you're already in pain, there is nothing to lose in trying a flush. Do you have the fresh grapefruit and good olive oil? Drink it down and hope for the best. Do you have anyone with you, to help you prepare and take care of you tonight?
If your gallbladder is beyond hope and the flush fails to provide any relief, at least you know you tried everyting before submitting to the knife.