Somewhat diverging from the OP's focus on weighing ECs along with academics in making admissions decisions for elite schools...
but in keeping with recent posts on flagging SAT scores:

This article provides some insight into the process of SAT scores being flagged by College Board and Educational Testing Service (ETS), although in this specific case, it may have been a result of intensive study and focused prep which raised scores. Ultimately, it was agreed that this student was going to take the test again, and that College Board would work on completing its investigations more swiftly to minimize impact on students.
Miami Gardens student drops fight to validate her SAT score
By Jason Hanna, CNN
February 9 & 10, 2019

When considering the possibility of surrogate test-takers, regular test-center ID requirements are followed stringently, therefore concerns that a surrogate took the test may tend to arise only when a test is administered one-on-one, as was described regarding the recent college admissions scam.

In the opposite vein, I am aware of homeschool students who took the Preliminary SAT/National Merit Scholarship Qualifying Test (PSAT/NMSQT) at a local public high school, during that school's regularly scheduled PSAT/NMSQT administration for their own pupils. The homeschool students were assigned to sit in a designated row and their completed, unsealed tests were collected together as a group. When results were available online, in reviewing them they were astounded to see some answers were indicated as erasures, and had been changed to incorrect answers. They were certain they had made no erasures on their tests.