So as it turns out, the LN uses Arc Titles that mostly differ from the WN. The Arc Titles are sorted by Volume up to Volume 8 (don't know what's beyond that since that's all I got to know from the LN vol. 8.5 Databook raws), so there is a discrepancy not only in the Arc numbers, but also in the Arc Titles. Many are still the same though. Anyway, here is the list:

Arc 1 (LN vol. 1): 地位向上編 Empowerment Volume (same as WN)
Arc 2 (LN vol. 2): 森の騒乱編 Forest Disturbance Volume (same as WN)
Arc 3 (LN vol. 3): 魔王来襲編 Demon Lord Attack Volume (entirely new)
Arc 4 (LN vol. 4): 人魔交流編 Human-Monster Interaction Volume (different from WN)
Arc 5 (LN vol. 5): 魔王覚醒編 Demon Lord Awakening Volume (slightly different from WN)
Arc 6 (LN vol. 6): 八星輝翔編 The Octagram Soars Brightly Volume (entirely new)
Arc 7 (LN vol. 7): 聖魔対立編 Saint-Monster Confrontation Volume (same as WN)
Arc 8 (LN vol. 8): 領土掌握編 Territorial Seizure Volume (entirely new)

So yea, we'll probably have to update things based on that. Also when I say "entirely new", I am refering to the fact that this segment wasn't an arc on it's own in the WN, but rather part of a larger Arc instead. The "different from WN" part doesn't refer to the content but rather to the Titles of the Arcs themselves, though if the name gets a change, then a content change is also very likely.

Regardless there is no longer any point in treating the LN as an "adaptation" of the WN at this point, as that is definitly not true based on the known facts.

EDIT: According to the Japanese wikipedia page for the series, we get all volume names up until vol. 12 (I think it should be up until vol. 13, but they didn't seem to have included it):

Arc 9 (LN vol. 9): 魔都開国編 Monster Capital Opening Festival (same as WN)
Arc 10 (LN vol. 10): 魔人暗躍編 Magic-born Behind The Scenes Volume (same as WN)
Arc 11 (LN vol. 11): 勇者覚醒編 Hero Awakening Volume (entirely new)
Arc 12 (LN vol. 12): 戦争前夜編 Eve of War Volume (entirely new)

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