Question: We're told that colleges consider only the highest SAT scores. But the report they receive from the College Board shows all scores. What assurance do we have, if any, that admissions people are in fact only seeing the highest scores? To put it another way, what's to prevent them consulting the full report if they want to?
Depending on the protocol at your child's target colleges, admission committees may indeed see all of the SAT scores, even though they may officially "use" only the highest ones. At some colleges, the secretarial staff will prepare a "tally sheet" (called by different names in different places) which includes the statistical information that will be part of the evaluation process (test scores, GPA, rank, etc.). Commonly, just the top test results go on the tally sheet, so the admission committees won't see the other scores. But, in many cases, the full range of scores will be on score reports in the student's folder or even on the high school transcript, so the decision makers will eyeball them, even if they're not formally considering them.
As you've noted, many colleges do only formally consider the best scores, even if from different test dates. Some schools, however, will use the best overall score but from the same test date. But, as you suggest, it's likely that they'll still get a look at all of the numbers and perhaps will be subliminally influenced at least a tiny bit, even if they're trying not to be. Thus, my advice is to try to avoid thinking about all this hairsplitting stuff, as best you can. It will just drive you nuts!
Note, too, that next year the College Board will introduce a new score-choice option so that students can decide which test results go to colleges. The ACT already allows this choice. Some folks applaud the SAT score-reporting change while others fear that some students will take the tests a gazillion extra times with the hope of bettering their scores, and this will give yet another advantage to the well-heeled who won't flinch at the extra testing costs.