When a Hentai Manga author gets one of his SFW works adapted to television, is he allowed to make pornographic parodies of the same?

Depends on the contract and the producer. There are no hard and fast rules.

The producer may decide that they don’t want other versions that would reduce value (or call negative attention) to the property that they paid for, so they might write that sort of restriction into the contract. Or not, depending on their views and how good their lawyers are.

Yes. They put Marge Simpson in Playboy. So anything is possible.