Not sure on the World Skills question, haven't played co-op. All three classes get Mark of the Keeper, so my guess is it only applies to the player who has it. It's really cheap to re-spec though, so should be easy to test. Most other world skills tend to affect enemies so would benefit everyone, unless they state otherwise.

Any bonus damage from gear adds to the base damage value, listed on the character sheet next to Health, Armour etc. All of the character's combat skills (left column of the skills menu) are then calculated as a percentage of that base value. The Marksman's basic attack does 20-40% of base damage, depending on level, so in your example that 50 bonus poison damage will be scaled down to around 10-20 per shot.

No gear adds directly to trap damage. Edit: I count runes as being different to gear.