For the top action, you cannot give Poison to an ally who is immune to Poison.
As with any ranged ability, you can trace the path to your target however you would like. It doesn't have to be in a straight line. For instance, the Plagueherald could hit all of these hexes with this action by tracing the path along the red arrows:
Also note that this is a ranged attack, so any attack against someone adjacent to you has disadvantage.
If an enemy draws two curse cards on an attack because of advantage or disadvantage, it still only suffers 3 damage since only one of them is activated.