Now Im reading that C's parents want EXCLUSIVE custody of baby L. Good!
I think there is nothing to be alarmed about here - it's just that the wheels of Family Law grind slowly.
* I am no greek law expert but here are some principles which apply most places*
We need to keep in mind the difference between custody and guardianship
Legal custody lay with B as the surviving custodial parent. In practice, given he is in jail, he no longer has physical custody. An application to the Family Court will typically be required to transfer legal custody to someone else. Obviously that will tend to be granted in such as case, provided the Court is satisfied the applicants meet the relevant criteria. That would end B's rights to custody, even where he later comes out of jail.
That leaves questions around guardianship. What rights does B have to input into decisions in the child's life?
I suspect it is likely the Court would also revoke his Guardianship rights, and appoint C's parents as the legal guardians.
In such a case, B would have no legal rights in respect of the child in future. Involvement would have to be agreed to, or he would have to go to Court.