The plot (or a system of fleetingly connecting sections of different plots in a day in the life of the City that Garrett is fleetingly visiting) makes perfect sense. Read the readables/hear the conversations/observe the details. Do not bother connecting everything to the basegame timeline or making a sacred cow of it. Thief at its core is best as dark fantasy which many authors nowadays trying to be faithful chroniclers of a non-existent world instead of creating their own enrichments of it ...
This mission may be a guilty pleasure of mine but I respectfully disagree. The level itself is actually pretty complex and I discovered that it's best (and probably meant to be given Garrett's situation and the slightly soured relationship with his principal) played entirely "revenge style", ie using the means you get that are actually very accurately dosed to be just enough (on Expert) to dispose of ALL enemies in the level in any ways imaginable in the Original Ghosting ((C) Civvie) style...
Rose Garden is not necessarily a good culprit here, I would say it's just as tightly packed as it gets for the amount of gameplay on offer. The coup is that it FEELS that way (ie big) at first by design choices. But yes, the prime notorious examples for unnecessarily big just for the sake of it are many and numerous. Looking at you, Feast of Pilgrims.
This is subjective but yes, AFAIR (spoilers galore) the woman in BF is the girl from EV, and EV explicitly takes place by in-game information as a side story right after L'Arsene, and both the stories (fleetingly) connect in EV in that we also meet L'Arsene there, so all take place in the same universe (in the above order on the timeline).
It can actually be counted as part of the (loose) L'Arsene series (L'Arsene, Emilie Victor, The Korrigans, The Den, this) by the same author that follows Garrett's tribulations with the key protagonists. L'Arsene is author's first but all are arguably classics. Special recommendations: Emily Victor and The Korrigans.