A letter (000F6A61) clips slightly into a table and falls completely through as a result.
000F6A60, 000F6A61, 000F6A62: Unique letters respawn along with everything else in the house. The respawn flags on each should be removed, assuming the frequent cell reset settings are intentional (see below).
Something I was wondering about is if the entire house was meant to never reset its contents. The bandits respawn, and all of the valuables (minus Chillrend and the Gray Fox bust) reset after about 10 days. Under many circumstances, quest related dungeons (or other locations) are marked as no reset zones. To be honest, it doesn't exactly make a lot of sense that the house continues to get restocked if Mercer Frey is gone (or dead). Though maybe there's some justification for this that I'm not thinking of.