Fanbook 8
During Myne's Nap - set in P2V3 during the spring prayer. I think it goes after Ambush, in the same gap as The Ambush and the Charm. Since Rosina's chapter ends a little after this one does - with Myne waking up at the end of it - I think Nap would go before Charm.
Bertilde's The Work of an Apprentice Noble - Set at some point during the preparations for the Battle of Ehrenfest, and definitely before Rozemyne was preparing to storm Ahrensbach. Though I didn't spot anything I could tie to a specific event. Looking for the best gaps in that section, I think between Preparing for War and Kamil's Baptism is probably the best spot, as there's a timeskip between the two chapters filled with Rozemyne brewing for the upcoming fight.
Rozemyne and the Knife - Ferdinand is said to be making the knife during Preparing for the Harvest Festival, and Rozemyne uses it in The Night of Schutzaria, but I don't know where she received it between those two chapters. Though it has to be before she left for the Harvest Festival, in Hasse's Harvest Festival. And with Preparing, Hasse's Contract, and Starting Merchant Activities all running on into each other, I think after Starting Merchant Activities is the best spot for it.
Hannelore's How It Turned Into Ditter. I couldn't spot anything in here that wasn't already covered by Careless Mumbles Lead to Ditter, so I don't think the omnibus needs this one.