My personal theory concerning the DNA evidence is that
Strong Sad is hiding it not because it holds the secret of his feet, but because it could be used to find out who his real father is - it's already been theorized that at least one of the Brothers Strong is actually a half-brother. One theory states that Senor Cardgage is Strong Bad's biological father.
I also think that the DNA evidence is actually a sample of Strong Sad's blood. I don't think we've ever actually seen him bleed, and since he clearly isn't human, why assume his blood is the same color as a human's?
Edit: I really hope you can open Strong Mad's closet... and see the
thing inside...