Can Niall rummage around in your character’s mind? Yes.

If yes, what stuff might he find there? Would he have difficulties? Everything. It wouldn't be difficult in most cases. Jesse's an open book.

Can Niall manipulate your character’s emotions? Yes.

Can Niall inflict hallucinations on your character? Yes.

Any other stuff you feel I should know? Jesse's weapon can nullify all of these powers if feels inclined to use it, which he probably would if he realized he was being manipulated.