2) No it isnt.
The Denarians are still restricted by both the Accords(which they signed) and the Rules of Heaven which have limits to how much power they are willing to display in public without freeing Heaven to take countermeasures, Mouse specifically scares Nicodemus, and Michael literally lives in the same city and visits Harry.
By contrast, Last Station is somewhere they can cut loose more comfortably, and I've seen what a Denarian feeling frisky can do with the right host. Its not pretty.
Last Station in particular shares particular geographic vulnerabilities by virtue of being underground.
By putting Lash on the surface, we leverage those elements of the Rules that Heaven enforces that prevents Nicodemus et al from doing shit like hulking out on live TV or attacking the White House in disguise to start a new war.
Y'know, the things we know they can do.
I also think you are strongly overestimating Harry's defenses. Firebombing his place works, we know that. Sending a couple of hit squads with machine guns would likely work too.