I wouldn't worry about soaking her food. The Epsom salt will relieve constipation and bloating, just by being in the water.
I would do the 50% water changes using Epsom salt treated water until the S-curve is gone and she's back to normal.
Simply predissolve 1 teaspoon Epsom salt per gallon of conditioned water, and use this for the water changes. This will maintain a dosage of 1 tsp/gal for the duration of her treatment.
You can leave her in this dosage for 10-14 days. Once the S-shape is resolved and she's no longer constipated, I would start to do the water changes using plain (non-Epsom salt treated) conditioned water. This will start to remove it.
Generally, I keep my fish in plain, conditioned water. I have one guy who is prone to constipation (and ends up with the S-curve when he does). When he gets constipated, I simply add a therapeutic dosage of about 1/8 teaspoon Epsom salt per gallon to his water. This generally takes care of the constipation issue. When I go the next water change, I use plain, conditioned water. This removes the Epsom salt.
Some people prefer to treat constipation with frozen daphnia. This is a great way to do it, but it also makes a mess in the tank. If you want to try that though, I'd suggest feeding it right before you do a water change. This will remove the uneaten food, so it doesn't foul the water.
Also, if you can get ahold of mosquito larvae, this is a GREAT food for them! During the summer, if I can find them, I'll rinse them in a little conditioned water, then use a turkey baster to drop them into the tank. Want to see your Betta become a world-class hunter? They track those things down amazingly fast.