Why does Atticus suggest they go to the porch instead of the living room, and why does Scout seat Boo in a chair in deep shadow?To Kill a...

That Atticus has Scout and Boo move out to the porch
rather than in the living room in Chapter 30 of To Kill a
Mockingbird, is evident, certainly, for the reasons already defined. 
However, knowing also what a timid character Boo Radley is, as well as the fact that he
has not been around anyone other than his own parents in years, may enter into the
decision of Atticus.  For, Boo is almost like an inmate who has been in solitary
confinement, and being seated in a room with four walls and in the midst of others may,
perhaps, be traumatic to such a man. 

By being out on the
porch, Boo does not feel confined, and he has both the opportunity of walking away if he
wants as well as the opportunity of viewing surroundings familiar to him:  his house and
the street that he watches the children play and walk near.  Thus, Atticus's action to
move Boo onto the porch underscores one of the themes of Harper Lee's novel that Scout
reiterates at the end of the chapter:

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Atticus was right.  One time he said you never
really know a man until you stand in his shoes and walk around in
them.