As for why he could switch forms when genetic strength are restricted here, David could guess that since the hybrid is already a part of him, the rules in this place couldn't restrict it.
He has four different forms which was the human form, the werewolf form, the vampire form and the hybrid form where he was at his most powerful, the combination of all three forms.
David's forms weren't a manifestation of genetic energy anymore, they were a changeable part of him now, Just a more flexible part. Although the restrictions within the sacred land was tight, it wouldn't restrict one's true body as long as you were human.
Grabbing the storage ring from the corpse, David checked what was inside and was slightly disappointed. The man only had a few hundred gold in his storage ring, with a few miscellaneous objects clothes, food and wine. Apart from those, he couldn't find anything else.