Great! That makes sense. I think all of the points you make about how the manananggal operates within Galina could be turned into a great hook in your blurb, while also satisfactorily answering the agent’s burning question of ‘how does this possession thing work?’
A final couple of super-picky points.
I would be wary of ‘selling’ your MS on the basis of its ‘originality’. Agents see THOUSANDS of submissions a year so the chances of never having seen anything like yours is unlikely. Agents get this ‘orignality’ claim all the time in subs letters. Given that it tends not to be true, it suggests that the author is not well read and / or doesn’t understand how well read the agent is! or… if the MS is TRULY original, then potentially it will be a nightmare for the agent to sell.
What do you want the agent to take from this line: “I also had experiences there that defy explanation.” It sounds like you are throwing down challenge to the agent, or at the very least inviting counter argument from them, when you want to be in amicable agreement.
I know I’m nit-picking now, but boy, do those agents make decisions quickly and the smallest thing can mean they pass.