A battery isolator and a solenoid are not the same thing. The isolator separates two batteries on a charging circuit. Current goes from the alternator to a central terminal, then out to each battery through diodes. (A diode is like a one-way valve for electricity, it goes out but can't come back). This keeps the two batteries separated so they can't drain back to each other. You don't pull current from the batteries through an isolator.
A solenoid is a high current switch that makes a connection, like linking two batteries + terminals or a battery to a starter. You can use a solenoid to link the barreties (in parallel) if you need more current (heavy demand like a winch, or self jump starts).
For your questions:
1)The marine switch enables you to choose battery A, B, Both, or Off. It serves the same purpose as the solenoid in linking the batteries for demand, can charge both batteries, but does not prevent one from draining to the other unless you reset the switch to separate them. It is also too large to mount on the dash, so it is somewhat inconvenient for self jump starts.
2)If I remember the diagram, it is about linking the two batteries together. The starter cable is still connected to your primary battery.
3)Add up all the amps drawn by all the loads you will use at the same time. Choose a fuse or circuit breakes just a bit larger (but check to see how much the fuse box can handle). That way it'll open the breaker for a short, but not for all the load you plan to use.
4)If you run an isolator, you will need some sort of solenoid switch to connect the batteries. If you run the marine switch, switching to "both" will link the batteries to jump start. But, you must also set it to "both" to charge both batteries, and then switch back to either A or B to prevent the two batteries from draining back into each other, or you must run the charging circuit through an isolator. A continuous duty solenoid will function the same as the marine switch for linking and charging batteries, but you can switch it from the any convenient location, like a dash switch. A momentary solenoid will link the batteries on demand (push button switch) but not when the switch is released (this is the jump start setup).
I use my spare battery to power aux lights and for self jumps. I run an isolator, a Ford starter solenoid and a momentary push button on the dash to jump. I think I described my setup in a post a few days ago here. It sounds like you plan to use your dual batteries for the same needs I have, so this would probably work for you. I think costs are about $80 total for all the parts.
