If King’s Landing (had) mourned its dead boy king, Jaime would never have known it.
1. If King’s Landing mourned its dead boy king, Jaime would never have known it. On the Street of Seeds a begging brother in threadbare robes was praying loudly for Joffrey’s soul, but the passersby paid him no more heed than they would a loose shutter banging in the wind...
2. If King’s Landing had mourned its dead boy king, Jaime would never have known it.
Hi.
What is the difference between sentence 1 and 2?

Thank you.Joffrey is the boy king.

Question

If King’s Landing (had) mourned its dead boy king, Jaime would never have known it.
1. If King’s Landing mourned its dead boy king, Jaime would never have known it. On the Street of Seeds a begging brother in threadbare robes was praying loudly for Joffrey’s soul, but the passersby paid him no more heed than they would a loose shutter banging in the wind...
2. If King’s Landing had mourned its dead boy king, Jaime would never have known it.
Hi. 
What is the difference between sentence 1 and 2?

Thank you.Joffrey is the boy king.

Answer

#1 is more focused on the present, and #2 is more focused on the past.

If King's landing mourned Joffrey, Jaime would never have known it.
This implies that King's landing is not mourning Joffrey, but if they started to, Jaime would not know about it (right now).

If King's landing had mourned Joffrey, Jaime would never have known.
This implies that King's landing did not mourn Joffrey, but if they (theoretically) had mourned Joffrey, then Jaime would not  have known about it (in the past).

Hope that helps :)

Answer

"If King’s Landing mourned the death of its young king" this would sound much better.
The difference is the detail.
 The 1st sentence gives the additional info about the beggar on the street praying'loudly for Jofrreys soul.
  But the passer by paying no attention to the beggar. They would pay more attention to open Door[shutter] banging in the wind than the beggars mourning.