dcarson is mostly right. The craft would have more fuel than the minimum required for one trip, and her added weight wouldn't make that big a difference. Same with food and water - the designers would plan in a fudge factor of at least 15% in case the trip took longer or some of the food was spoiled for some reason.
The seat, though, is a big problem. Fighter pilots in World War 2 might experience a 4.5G force in a powerful and agile plane in a max turn. A lot of them would lose blood flow to their brains and "gray out" in that situation. Astronauts can experience up to nine G force during liftoff. That's survivable only with an acceleration couch and a padded flight suit. Lois would have exerted a max force of about a thousand pounds on that jump seat, and because she was sitting up she would have suffered spinal damage and impact injuries similar to a fall from a three-story building when the jump seat collapsed.
If Superman had flown up to rescue her after the shuttle achieved orbit, it's likely he would have returned carrying Lois in a body bag. So, in retrospect, she's lucky Luthor tried to blow up the shuttle.
