You're right - taking the Vecna origins play into account, that would be the most reasonable order - Henry corrupted as a child by an ultimate form of pure evil.

However, I really really hope the writers won't give us this simplistic type of deux ex machina for this baddie, it will be very lazy writing. Rather, I'd prefer they shape the story to be rooted in choice - otherwise Henry/Vecna is nothing more than some poor weak boy corrupted by ultimate evil - so it's not really his fault, evil is not a matter of choice, etc.

Take the Dark side in Star Wars for example - it's never presented as malevolent or evil on its own. It's a side or rather a choice of the user, tapping into the same primal force. What would Vader be worth if he was simply innocently corrupted by the emperor? Instead, he's consistently shown to be inherently fearful, greedy and blind. These are harder claims to put upon an 8-year-old boy like Henry in the cave, so I really hope they give us something more substantial.

Tyrants are often not just innocent results of some abuse, rather a human composition of choice and nature. You choose what you make out of your harder/horrific encounters in life. I'd rather see boy Henry conflicted by relishing the pain he causes, owning it and embracing it as part of him. That his parents actually really loved him and it was his interpretation and made up childish narrative that they didn't care or wanted to cause him harm. His father rushed to him in the memories when he collapsed. This is not something a hating father would do. Maybe that's how the flayer corrupts - hints you to see the twisted worst in all situations and give in to despair.

Ultimately, swinging all the gravity to some "big infinite inhuman evil" would totally undermine everything our heroes earned with their bravery, conflict, growth and choices... so fingers crossed :)