The opposed check challenge die/dice is instead of the difficulty, not in addition to the difficulty, correct? The plus symbol makes this a bit confusing.

Yep.

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Yes, melee and ranged attacks have a base diffculty of 1d, INSTEAD the difficulty can be determined using the opposed checks table from the rule book.

Brilliant, cheers.

yes, p.58 rulebook : you may use opposed check instead of the flat 1d difficulty in combat.