One of her opening offers was to make him the single best human water mage on the planet, don't confuse Harry turning down her offers for her having nothing to offer.
Maybe she could have, or maybe she was lying to Harry. He admits picking up the Coin because of the power, she may have been playing to that.
You mean the host pulling out displays of power and precision that Harry said would be impressive even by senior council standards? That host?
Jim said that Hannah wasn't a wizard because all she could use was fire, but she was very good at it. Lasiel improved on what she was good at, but Hannah didn't display much else. Marcone on the other hand had absolutely no magical talent and Thorned turned him into someone who could use advanced magic that Harry hasn't displayed (teleportation, molecular manipulation)
Yes Thorned and Harry would be a nightmare combo (he's also Jim's second pick for a coin to stick Harry with after Lasciel).
I'm not saying Lasciel wouldn't have made Harry more dangerous, I'm just looking at what she did to Hannah's abilities vs what Thorned did for Marcone's. Lasciel improved Hannah's power while Thorned apparently gave Marcone abilities he never even had, and at a very high level. Lasciel may have brought other weapons to the table for Harry, but from what I can tell, pure wizard power would come from Thorned.