1. He managed to leave the mansion after giving to death (If you remember, he tried killing himself in the lake and that meant he gave up being an immortal, and it was he's master who brought him out of the lake). Clodan is no longer an immortal
2. The others are already back to normal (Thats why Della could finally die, and Mary, as eel as the gardener's face came back to normal). As for Ray, he already gave her the potion that will bring her normalcy. Go back and read the chapter where Mary handed her the letter and a bottle of potion.
3. Clodan will always have a beast within, he is a sorcerer and that comes with sorcery. (He's curse was immortal and the pain that came with it, hence why he was mad ho*** at times). The snake is like he's spirit animal, if you get what I mean.
I hope i answered your questions properly. You can as well reread to see the parts you missed.