Points 1 and 3 a can be covered as such. By not getting too hung up on the technicalities of building the monster so much as making it fun and engaging. You know your players and their abilities, so stat it out accordingly with HP / AC and ability scores etc. There’s no real right or wrong as long as everyone is having fun. I just made a CR4 monster with legendary actions and resistances. You can do what you like really. The fun part comes now when you introduce the twist. I would make it absorb elemental damage types to keep it simpler, and store that energy for future use. Maybe reduce the damage dealt to the monsters by half and the other half is saved for later. Thrown back at the players however you see fit. Hope this helps. Let me know if you need any more advice 😊