I don't agree with the idea of Harry being lazy. Maybe he didn't work on veils as much as he could have, as he was not very good, and worked on his strong points. But he got better. He does not need his blasting rod as much as he once did. He appropriated Luccio's laser after he saw it in action. There's a fire refinement Ascher could have used - with the same power and a smaller cross section, she punches THRU Harry's shield, instead of it being reflected. He worked nights and weekends for... 2 or 3 months, was it? for that forcefield crystal Molly used on Demonreach. Eb seemed impressed. Even Mai did not seem sure she could breach it. Little Chicago was not simple, and took a lot of time and effort. Not a lazy man's tool.
We see how Ascher was massively boosted - she was burning Wardens from within, and the finger torch Harry said was, what, a century off?, while the Gatekeeper said Harry was the most powerful AND TALENTED of his generation - both terms. She was taught by Lasciel. Some Fallen may give their vessels/vassals more power - also recall when Nic offered her coin to Harry, Harry asked why he should believe Nic would let him leave. Nic replied with the coin 'I doubt I could stop you" - as in, with Lasciel providing as much info as she does to Harry he could blow Nic halfway to Wacker Drive if he wanted. Nic may not be hurt, but he'd then be in no position to stop Harry from strolling out the door. The Fallen were angels... and Harry seems to imply in Battle Ground that if they could trick the enemy army into coming too close to Michael's, Michael's security force would blow them away, as if it is no contest. So, angels are hugely powered. So, maybe most hosts only give their vassals 10% of what they can - what if Thorny and Lasciel give 25%? Their coinholders would be much stronger. And Thorny and Lasciel seem to be some of the thinkers of the Denarians, unlike, say, Magog or Ursiel. And you match up the coin to the recipient if you can - you want a good partnership. Magog is force and smash, Ursiel seems similar - plus a genoskwa is probably not high on potential for magic. Harry and Marcone are thinkers. Well, Harry, when he stops to think.
You only soulgaze once, right? The Marcone soulgaze was before we see any coins... and it did not even tell Harry Marcone is probably not his real name... he may have some small aptitude. Harry could have told if he was a practicioner, but maybe not if the talent had withered .. to be revived by a coin. His teleportation.... maybe it is just a different, temporary method of making a Way. A localized time-space warp. Picture a flat map. NYC to Maui is... 6000 miles, say. Fold it, they touch. He folded space. Nice trick - and maybe he is only really good at that one trick. He learned one really well, the others are crude. 10,000 hours to be a master? Well, maybe he spent 1000 on one little trick....
As for Merlin and Demonreach being impossibly magic-dense? Um, maybe impossible for HUMANS, recall in the myths, Merlin is usually only human on his mother's side, his father an incubus or spirit. Our rules may not apply.