divers are the bomb, just use your first eyelit as a measurement from your reel, from your reel grab your line, pull it out to your first eyelit. probably about 2". until you get to the depth your wanting, the diver will not be strait down so my rule of thumb is 3 pulls = 2.5 ft, i wouldnt use a fender, some might, i would throw on a dodger about 12" behind your diver then throw on a 2.5" leader.
well depending on the fender it could trip the diver, divers work by diving down, when a fish hits it, its trips up the flow and "unlocks" persay. when trolling, any change in speed with the fender on it could trip it, not sure, never ran a diver and a fender, give it a shot
There is no replacement for downriggers. You can try alternatives but in the end they can't be beat. I fish four rods on my two downriggers and would never go back to the alternative. You can pick up a used pair of manual DR's for under a hundred bucks if you shop around. That means you will be fishing with nothing between you and the fish but a dodger which will not impede the action like a diver will. If you don't care about experiencing the fish on the end of your line by all means go with the diver.