Corrected entry: We are supposed to believe that an accidental shuttle launch put a bunch of kids into space. At any time during the launch, however, the mission can be aborted by separating the shuttle from the external fuel tank and solid rocket boosters. This is an abort procedure used by NASA. The shuttle can either turn around and return to Florida, depending on how close it is, or land in a backup facility in Europe or Africa. This procedure can be implemented before the throttle up procedure, which is depicted in the movie and indicates the pilots are committed to completing the flight into space.
Correction: It is true that a Space Shuttle can abort a launch, it cannot do so while the SRBs are attached and firing. Because the SRBs use solid fuel, they cannot be shut down after being ignited and must burn out before separation is possible. After that, the fuel tank would remain attached to the Shuttle in order to provide fuel needed for an abort sequence, and the Shuttle's main engines would be used to either change velocity and get on course to a landing site(RTLS, TAL), or to get them high enough to perform one orbit and land (AOA), or to get them to a low but stable orbit (ATO).
Corrected entry: While in space, Max asks for Kevin's help to show him how to use the shuttle washroom. Kevin takes Max into the washroom and shows him how it works, then tells Max to close the door and turn the fan on. Only a second passes after the door shuts until we hear Max "moaning". He couldn't have taken his space suit off and the other preparations in under two seconds time.
Correction: You're assuming that his 'moaning' is due to him relieving himself, its probably more to do with how he has to use the suction device on himself, for a 12 year old boy, that would probably be a big deal.
Corrected entry: They use Tish's belt to make the suit smaller - a belt which is only a couple of feet long - but they manage to wrap it around Max about a thousand times.
Correction: In actuality the Campers or the astronauts in the shuttle all had similar belts. So by connecting them all together in one long strap they were able to wrap Max up in the spacesuit prior to his trip to Daedalus.