I was thinking about keeping Jan at the ends of the mat hiding behind Leebo and with the Dash crew card. Can she shoot through Leebo. Since Dash's ability says: your attack cannot be obstructed.

Shooting through ships doesn't obstruct attacks. Only shooting through obstacles obstructs attacks.

Tactical Jammer on a Large Ship will obstruct enemy attacks.

To address the other misunderstanding there: the Dash crew only affects the ship he is on.

Also, an obstructed attack does not mean you can't make an attack it just means the defender gets an extra defense die.