i don't use a filter sock. when i did, i used an 8" one, and changed them out every couple days. what you have there looks to be a 4" sock, correct? (the diameter i mean)
put an elbow on the bottom of the drain. see if that stops the raging water and salt creep some.
no, those power heads aren't too much. arrange them how you need them to be. there is no exact formula for this. just try to accomplish the least amount of dead spots possible.
as far as the noise is concerned, if you have a tube slightly larger than regular airline tubing, that should work. usually the airline tubing works for me. you can see it on many HOb
overflow boxes. some of them come with a piece of rigid tubing with an O ring on it so you can adjust it up and down to quiet the box.