A note on part 4: there is probably a joke being lost in translation.
Now, I can't say for sure, but I have reason to believe the author was on an AquaPlus binge while writing this series, and a lot of characters are named after characters from various AquaPlus works. I.E., Melvy is named after Melvy from Dungeon Travelers 2, and Kuon is named after Kuon of Tuskur (This makes more sense later).
Anyway, the point is that Sylphie's name is mistranslated and should be Silfa (I translated it Sylphie because it's supposed to be short for Sylphonia).
The joke is that Sylphie the princess is great at housework and wishes she was born a maid, while Silfa from To Heart 2/Dungeon Travelers 1 is a literal maid robot who happens to be completely incompetent at her job.
Unfortunately, I never played To Heart 2, and Dungeon Travelers 1 only released in English last month after a decade or so in stasis. It only really clicked for me around Dark Cathedral.
This slipup does not alter the story whatsoever, it's just important for understanding this one very specific reference. Thank you for reading, and I'm sorry for taking up your time.