If you activate the card "Unpossessed" during the Damage Step, it can be activated from the start of the Damage Step until before damage calculation.
The effect ""Charmer" monsters you control cannot be destroyed by battle." does not make a Chain Link.
The effect "If a "Familiar-Possessed" monster you control attacks an opponent's monster, it gains 800 ATK during damage calculation only." does not make a Chain Link.
The effect "If a monster(s) you control is destroyed by battle or card effect: You can Special Summon 1 Spellcaster-Type monster with 1500 DEF from your Deck, whose Attribute is different from at least 1 of those destroyed monsters' original Attributes on the field, in Attack Position or face-down Defense Position." makes a Chain Link. (It does not target. Also, if you activate the card "Unpossessed", you cannot activate the card and its effect in the same Chain Link. If "Unpossessed" is already face-up, this effect can be activated even during the Damage Step.)