I’ve never heard of an audit where the student wasn’t allowed to exclude the FA money because it was co-mingled, but I can’t tell you it never happened.
I had no issue with excluding the amount in D’s account that was left over from FA (in her case it was loan money carried over from year to year). It was in her regular checking account and was less than the total amount received.
What you can’t do (or aren’t supposed to do) is spend all the FA money you’ve saved and then ‘replace’ it. If you have $10k in excess FA and at some point your account is reduced to $5k but then you save some other money (income, gift, lottery winnings), THAT money is not excess FA. If your FA savings goes down to $5k, that’s the amount you can exclude as FA.
If you can look at your account records and see that $10k came from the school/outside scholarship on Sept 1 and the balance never goes below $10k, you shouldn’t have a problem showing that $10k is all FA and can be excluded.