Going out on a limb here.
What if Monica turned up at Nathan's to pay him off, just as Gideon was leaving, he doesn't see Monica . Monica finds a dead Nathan, calls Clancy for help. Clancy arranges the hit on Yedemsky, care of his brother(job for life care of Stanton). Hence Clancys comments, "you'll be ok" to Eva.
It was Monica's comment to Eva at lunch, when Eva said she and Gideon didnt have secrets, Monica asked "what about him(Gideon)?
Then Monica went on to say, "everything I've done is for love. Everything"
In chapter 17 Monica s definitely hiding something big.
Or, what if Monica was just leaving after paying off Nathan, she saw Gideon, he didnt see her. She waits to see what happens. Sees Gideon leave after killing Nathan, then calls Clancy, for help.
Tying myself up in knots here!!