The projectiles sticking around is a good idea. They could act like spikes on the floor/walls, hurting the player that walks into them, then disappearing (because the player crushed them). Of course if the projectile touched the player before touching the floor or walls, it could just freeze the player.

Sprite rotation would do more harm than good. We had a similar idea, but changing the shooting direction would make the game unpredictable (tiny changes in the floor's slope or moving objects could throw off the precision of shots).
We're still thinking about an analog aiming mechanism, but that would most likely be out of place as well. Maybe in another version.

The seekers will be able to rotate in any direction, there are no sprite movement limitations in Unreal 4.