I've looked at this topic at least three times now and the fact that nobody has answered is starting to make me feel bad. :( So I will answer!
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1. Since some key items appear only from respawned (...) objects: what would be the wisest method to gain them?
That's really up to you. If you decide to pass turns, you can trade your time for EXP, which is always nice. If you decide to fight, though you'll lose Vitality and Durability, you'll get more EXP and weapons from killing more monsters. So it kind of balances out in the end.
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2. Is the Japanese wiki regarding items mostly complete so far?
It seems to be. I haven't found any missing information on items there yet.
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3. Are there any special/secret scenes or unique items/weapons which require a certain knight (Transoul <.<)?
I don't think so. The closest things would be the slight ending alteration based on a certain knight, and all of the little Pause Talk sequences.
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4. Is there any new content to explore in the higher difficulties (extra scenes)? If yes, about how much (not so much, decent, very much)?
I don't think there are, but I'm not 100% sure yet.
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5. What would be the best way to use up as little as possible vitality? I really hate it when I need to sacrifice allies although it is part of the concept.
To maximize your damage output, really. For normal enemies, always take advantage of their elemental weaknesses. For bosses, pay attention to the damage amount on weapons when you're selecting what to use. Don't rely on the weapon's level to tell you how much damage it'll do, because there are a lot of low-level weapons that do greater damage than higher-level weapons. Also, try not to let enemies regenerate HP too much. The recovery rate of later enemies can get pretty fast, especially on higher difficulties.
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EDIT: 6. If I skip a turn: should I pick EXP or MP?
I usually choose EXP unless getting MP is annoying on the Scene I'm on. It feels like you get more EXP for your time than MP.
Hope that helped despite all of the things I wasn't sure of.