Have you submitted just one part of the app to Amherst? That seems unusual. If so, your option is to either withdraw the app, not a great idea IMO, or contact the admissions officer representing your region and ask them to add information to the part of the app you submitted. If you withdraw, then submit the Common App instead, that’s just going to raise red flags, IMO, but maybe it will be fine.
This all sounds odd though. So you are planning to take the SAT and ask them to add that information to your ED app? You aren’t going to get your score back in time. There is a due date for the app. Test scores will be part of that due date. You can’t just keep contacting them asking to submit more information.
And when are you submitting the supplemental essays? Why submit a partial app? There’s a lot of confusion in this post so please clarify.