How did Welfin know about Komugi? In episode 134, when Meruem asks about Pouf's secret to Welfin, Welfin thinks so hard, harder than his brain is capable of. And the result is that he says "Komugi". 



Why did Welfin know about Komugi? Did he meet her before?
Just recently re-saw the arc so it's pretty fresh.  He knew because Pouf had him call Pitou to tell her that Killua killed her.  It struck him as a weird request at the time, but when he was racking his brain for the right answer, he started connecting all the dots and realized she was the keyword in all of it.

He most likely figured out Komugi was the right name because he put the following things together:  


The King is looking for something.
King wants to know what Pouf is hiding.
Pouf used Welkin to lie to another Royal guard.
Pouf lied about the enemy killing a person named Komugi who was in the enemies' possession
Royal guards are incapable of being un-loyal to the King. 
Ikalgo/Palm believes they have something the King will definitely trade for.