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Without schematics in hand explanations of "How I installed" somwhow gets lost to me at times. Generally speaking the motor wire harness comes directly off the battery and the engine has its own main fuse inline. No circuit breaker in that chain of use. The "on" position for the key then goes to the instrument lighting and some functions. Usually a purple wire. My Whaler had a separate switch for the fuel guage senders but I made it part of the key "on" position. I also have electric trim tabs that are tied to the key "on" switch so they automatically raise in the off position (safety feature when loading onto a trailer). There can be other accessories linked to the key switch but........all those accessories are linked with the on position of the key and do not interfere with the harness. With an outboard motor the throttle/transmission handle in the neutral position is the completion of the negative circuit to allow firing. A side note.....my inboard motor of my Penn Yan has no relation to the negative in the throttle/transmission lever (it could if I desired).....my transmission has mechanical linkage off the side of it that completes the starting circuit. Anyway the outboards have built in protection and you should locate the main fuse in the motor for servicing.....carry a spare fuse. My original 1981 235 Johnson had an odd ball fuse in line.

I would just be repeating myself for coming off the battery. From a personal standpoint I prefer fuses and circuit breaker switches over the "push button reset" style. I do have a 4 gang 3 position (on-off-manual on) Blue Sea circuit breaker switch panel but those are for my bilge pumps and I have a 40 amp breaker switch servicing that function. Those are hot wired off my battery and bypass my Blue Sea Dual Battery Plus ACR switch. So....to answer Bill.....
1. I guess so.......I am not understanding "breaking original circuits". All you are doing is adding a main breaker switch.
2. You cut all power to accessories......motor is its own animal
3. I do not know.
4. Motor independent.....by all means cut the service to a smoking panel
5. I am not big on reset button breakers.....personal choice.....on an electric crab pot puller the switch has a reset but there is a circuit breaker between that switch and the battery.

edit: to better answer question "5". No......you are not necessarily protected by fire with a circuit breaker (a circuit breaker acts as a switch) by the battery (s) which you rely on to automatically trigger with problems. Know where it is to manually shut down the power (big reason I do not use reset button breakers). The higher amperage for setting it off protects the wire/cable to accessory panel. The correlation between wire sizes and amp use for accessories to prevent heat. Example: (this is an example of poor wiring but the premise is the same if you have the correct wires that happen to cross) you install a washdown pump but to save work you splice into navigation wires. Navigation wires 18 gauge or there abouts........washdown pump 10 gauge or 8 gauge. You flip on the the washdown pump and the amp draw cooks the small wires. Too far away from a circuit breaker at the battery. Hopefully you fuses or breakers go off but.....that massive amp draw may very well burn/melt set on fire a pinched or flawed area in the wire before the fuse melts, quick like a "jack rabbit". You want to be able to immediately to kill the positive circuit. My 1979 Whaler Revenge has several circuit breakers (anchor winch, bilge pumps, electronics, house/start, 2 more for downriggers) however......I also installed my dual battery switch below the dashboard at the helm. If I smell burning wires or see smoke......battery switch to the off position. With the center consoles an additional switch (Blue Sea makes a flush mount breaker switch) could be installed on the dash or, if too tight and crowded, one installed in the console that you can reach under and flip. Or one by the battery you flip immediately. A nice levered switch that you know where it is.

For anyone owning a boat or purchasing one.......do check out and become familiar with what you have electrically. Going out for the day with the only knowledge that the key starts the boat and the navigation lights work after you flip a switch leaves you in a compromised situation. I am a registered Coast Guards Safety Drill Instructor which allows me to run the required safety drills for commercial fishermen. Once a month requirement. My electrical was installed way before I became an instructor. I do keep a cheat sheet on board identifying where safety equipment is and what it does. I also show passengers the lay out of my boats before I leave port in case I am busy or incapacitated.