If the person isn't "a good fit" then yes, as long as the record (your emails) supports that decision. Email saying things like "he's so much older than the other applicants" or "he's got school-aged kids, so his head won't be in the game" will be found in discovery and will lose your case.

Another thing to consider: people who file discrimination lawsuits usually do not prevail on the discrimination claim, they prevail on the retaliation claim. It is often quite easy to show retaliation, especially if it is an employee and not an applicant.

Yes, it is often easy to circumvent the law.