I made that sort of a suggestion before, except that one also had an AoE effect. Not quite a sniper but eh.
Weapon suggestions - Pulse Cannon and Krypton Phaser.
Possibly. Though I think that’s not that much of an issue. But having a precision weapon without an area effect would slow down the gameplay too much (having to pick off enemies one by one on more complex waves probably wouldn’t be too enjoyable and would take too much time) imo.
Yes, Photon Swarm at 
0 is just slightly less powerful than at 5 as long as you use manual fire (up to 2600 damage per second without any chance for overheating as far as I remember).