Sorry for such a long winded response. Wiring is very challenging till you get a good grasp on it.
1) I personally run constant power to any accessory fuse/relay blocks but this is a personal preference thing. I also like to relay every circuit, while it is a bit overkill it keeps the load off the switch and they typically last a lot longer that way. This also allows me to run one bundle of "signal" wires to switch location from the relay box. That allows me to power the switches and switch lighting under the dash to key on, constant, headlights on, whatever. If I want an accessory on key on only power, then I just tap key on power to the switch and the signal wire makes its way back to the relay box under the hood. I personally would consider doing something like this instead of that fuse block you are looking at.
2) yes and no, you could run a fuse but the circuit breaker is nice if you were to trip it, you can then sort out the issue and reset it instead of having to replace a fuse.
3) you should not need a circuit breaker for key on accessory power to control a relay or solenoid if you are choosing to power the entire fuse box off key on only. A fuse will do the trick, typically a relay or solenoid has such a low amperage draw for the turn on that its almost totally unnecessary to use a circuit breaker. Also if you are running a decent amount of accessories instead of a relay you probably want to run a continuous duty solenoid to power the fuse block.
4) the spare spots on the fuse block just have spare fuses, there is a spot between the 30a main and 30a spare though that appears you could insert some packard connectors in. That would allow you to put a fuse in there to power the solenoid/relay if you wanted to do key on only power. At that point you would also want to replace one of the factory packard style connectors for key on wiring thats right there and add a jumper over to your new fuse. That would be the cleanest and long lasting way to do it IMO.
Another thing to consider is the grounding side of your accessories, its very nice to wire all your accessories positive and negative side to one location. You can add a ground bus and locate it right next to the relay box, this would allow you to have all your wires in the same location for positive and negative on any accessories. Another thing I like is if you are using relays then you only have to run your accessories to under the hood, no more having to go to the dash and route wires, etc.
There is a really good write up on it that i am trying to find and will share if/when i find it.