He'll need help for the lights and electricity, but plumbing(even hot water) is old tech and not something that he'll hex.

And as I said in another thread, Harry probably has a salary as the Winter Knight, he's just been too stubborn to ask for anything from Mab. Once the thought hits him that it's part of the deal he already made and his just due, it can most likely cover his expenses at the castle and the diamonds will do the renovations. Molly has an eight figure expense account, Harry can make do with a seven figure one.

I think any way to Demonreach would open up near the Outer Gates probably on the outside near them. It would explain how Rashid knows a way and could get there quickly. in TC.

The way Stars and Stones was used made me think of Stonehenge, which was used for astronomical purposes. Perhaps something like that is a Stone Table for Starborn.

Stars and Stones is probably a Biblical reference. It was hinted at in CD when Harry caught the cracked jar with the name Wormwood. Wormwood falling from the heavens in the form of a disease carrying asteroid would be a disaster on the right scale for the second act of the BAT.

Think about it.  I would figure out how Mommies curse worked to keep Papa Raith from feeding and develop a charm to do that while I performed the horizontal mamba with my newlywed.  Effectively slapping a muzzle on her. I mean, he's inventive right?  He knocked up a proto vampire using unicorn rope.

Well since Leah was able to put the Red Court predator to sleep casually, She or Mab or Molly could give it to them as a wedding gift or an enchantment or their wedding rings.

I think you guys are focusing on the wrong halves of these partnerships. Nicodemus is a big threat  but that is because Anduriel is partnered with him. Even bound as they are, the Denarians are some of the powers of the Dresdenverse. The brutal thugs are near peers to the elder powers of Summer and Winter. Their direct influence is limited by their mortal partners but they are still Fallen Angels will all their knowledge and power. Nic is merely the most competent and cooperative host they have acquired. By dint of that he is the leader and front man, but the Fallen are the ones ultimately calling the shots.