Varner likely knew Guilliard was gonna hit the ground in a bad way but by that token its Melvin's responsibility to defend and protect himself. Melvin put himself in a risky position in order to try and obtain a more advantageous position. Your suggesting that Varner should allow Guilliard to get that position because defending it would either force Melvin to release the position or take a dangerous impact.
Agreed, Melvin was only in as much danger as he allowed himself to be. Obviously by the outcome of the slam, Melvin, even without letting go of Varner, was still able to tuck his head and avoid being slammed. That alone shows he was in control of the situation. That and if all ease fails... he could have always let go of Varner....
No rules were broken and no dangerous conduct was seeing this day.