Do merconaries can increase rarity?

Can mercenaries wearing gear that increases item rarity affect it?

1. In the case when they kill enemies themselves

2. In the case of applied "Gravebind"

3. Do mercenaries have a base decreased or increased item rarity modifier?

I assume they do, because they mentioned they add to party bonuses, at a scale of having .5 of an extra party member. I don't know for certain though.

Yes gravebind works and yes you can increase magic find. Just remember that your mercenary has to be close to the enemies for it to work and if you’re zooming through the maps it will most likely not work since your merc is behind. Works for tough rates and bosses tho