You are taking general statements about very specific peoples powers and then applying them piecemeal nilly willy. That doesn't hold water.
No, I'm taking evidence we've seen and been told directly about taking and absorbing power from one being to another and applying it to a suggestion that Harry take and absorb power from other beings into himself.
What's preposterous is ignoring all the evidence and WOJ and people directly telling Harry that even touching the power of Demonreach is a terrible idea, and saying everything would be fine.
Harry can't even do specific things with his own power without being affected by it.
Every creature in Demonreach is vastly more powerful than Harry individually. And you think that Harry can absorb
all of them without being affected by it.
That is just plain not how the world of Dresden has ever worked. You don't gain power as simple as plain knowledge without being affected by it.