Justus's An Aged Board and a New Letter - While it says its set between P4V9 and P5V1, it contains a letter Rozemyne wrote in P5V1. The earliest I can find her sending Ferdinand a letter (as opposed to her complaining that she hasn't had a chance to yet) is in Hirschur's Personal Librarian, but if I've missed a Rozemyne->Ferdinand letter between there and The Fellowship Gatherings (where Detlinde tells Rozemyne about the 'love song'), do let me know!
Barthold's Hidden Wrath - This one looks like it ends after the early purge reports from Ehrenfest (i.e. when the restrictions on the FVF students are first loosened, but before the Archducal Couple's visit when they know who actually needs to give their names). That would place it after the chapter Passing Classes in P5V1.
Loyalitat's Sneaking Suspicion - This one ends with the investigation into the Bride Taking Ditter match's ending. I think the best spot for this is after the P5V2 Epilogue (which contains Hannelore's POV of the match's aftermath).
Anastasius's Various Goals - This one seems to come between Detlinde's performance on the whirling stage and the meeting with Rozemyne about it - placing it after the chapter Detlinde's Dedication Whirl.
Lutz's Tuuli's Worries - This is set in P5V4 before the official start of Spring (i.e. before Changing Surroundings and the Feast Celebrating Spring). I couldn't spot anything more precise as to where to place it, so I'm torn between immediately after the Prologue and in the gap after Brunhilde's Suggestion, next to Color-Mixing and Engagement Ceremonies...