This card is not restricted by LOS, correct?

So I could theoretically throw it around a corner that im not even touching?

Grenadier doesn't say anything about LOS, so it's not necessary. You simply use the rules for counting spaces.

Choose a space within 3 spaces. The distance from the figure to the chosen space must be 3 or less, nothing more is required.

Can you count spaces if there is no tile there? Say 2 parallel corridors separated by 1 tiles worth of dead space?

You cannot count spaces through walls , doors, blocking terrain spaces, or blocking terrain edges.

You also cannot count spaces through corners where any combination of wall, door or blocking terrain meet.

Many effects require players to measure the distance between two spaces. To determine this number, the player counts the number of movement points it would take for a figure to move from one space to the other.

Impassable terrain, figures, and difficult terrain can be moved into and through without costing additional movement point for this measurement. This measurement cannot go through walls, doors, or blocking terrain.

· If a figure's ability specifies to choose something within a set number of spaces, players count spaces from the figure using the ability. Likewise, if an ability specifies to choose an adjacent figure, the figure must be adjacent to the figure using the ability.