The Merlin is capable of making a ward that held back the entire red court of vampires, so the RPG's statblock of him is severely underestimating his power, and likely every other senior council member. I mean, OW gives the Nightmare more refresh than the senior council, and that can't be right.
Harry has at least -20 end of changes. He's got two forms of sponsored magic, marked by power, probably a couple of refinements, and several of the inhuman level enhancement powers.
I just checked Ancient Mai and the Gatekeeper, and the book estimates them to be in the upper teens. I understand that this is a starting point and that people are free to change it up to suit their games, but the book puts the line for High Council membership around that point. Which is what I remember it being roughly, and what I was talking about above when I say that you can hit that level in your game.
Those are way underpowered and stated to be so. Try adding in Listens-to-Wind's shapeshifting and you get a low-30s character minimum. He seems to be typical of them in powe level.
As for Harry, I don't think he's in the 20s as of Changes. Assuming that 1 book is 1 story, that gives him a point of Refresh every few books. If it would be every 3rd book, he's around 14. If it's every other, he's around 16. So I'll call it 15 for a nice middle road. So he's powerful, but not quite on par with a High Council member.
He gets a Major Milestone more books than not, I've got him at 18 post-Small Favor, but let's go with your 15. He, of course has -14 worth of powers and Stunts at this point. Then comes Changes, where things, well, change.
Becoming Winter Knight grants -8 Refresh of stuff (Marked By Power, Unseelie Magic, Inhuman Strength, Speed, and Toughness with a +1 Catch) and sends him to -22 worth of powers. This will also shortly send him into NPCdom, but he is a low 20s Refresh character until then.
Then again, I play a lot of Exalted, and I like Nobilis, so the idea of the PCs becoming some of the most powerful things in the setting doesn't frighten me. I sort of like the idea that -- after a year or two of regular weekly games -- the PCs might be in line to join the High Council, or maybe be able to take the Council as part of their schemes.
Sure, and I've pointed out th lack of needing to have the same Refresh level to do that.