No mate lets answer your questions in order.
1. No just inserting a relay will not increase your voltage to the pump(s).
2. Yes you will have to run cables from your batt to the relay to supply your pump.
3. You will need a 30 amp relay, wont make any more voltage but will be a lot more durable.
4. Your in line fuse can go either before or after the relay, it will not make any difference. And your fuse is not going to worried by voltage, its the amount of current running through your fuse that will melt the fuse if you have a problem. I use a 20 amp fuse for each of my 2 fuel pumps.