Additionally, the WC is a signatory to the Accords.  Harry wielding Winter power to attack members of the WC might be construed as a violation of the Accords.

I'd imagine it is rather dim in the spire.  Isn't that the reason that they all wear goggles on the airship?  I thought that when they went outside one of the characters forgets and is stunned by how bright everything is.

I think you may have hit the nail on the head about there being cases Anna didn't know about.  It's also possible that 20 and dozen are rough estimates. It is possible that some of these victims were Isolated by circumstances even though they did not live alone.  The two with housemates could have been living with roommates just to afford a better place, or living with close friends but working lousy hours so they were never home/with those people.  It's also possible that the Skavis chose to attack at a time when people were separated temporarily, like a husband being out of town for business.

As to the Skavis blending in, I would guess that he probably set up a few of these personas in those major cities, and then just started killing after he had gotten people comfortable.  For a critter that can live hundreds or thousands of years, spending six months or a year bouncing back and forth to major cities every few days wouldn't be too hard. 

I think the ten minute warning was how long Elaine had said she could hold the veil.