Firstly, the feed to the cameras:
I think, to be safe you should use a low drop-out regulator to feed your cameras - this takes care of slight overvoltages. The KA278R12C is a linear voltage regulator with very low drop out: -
Note that even when the input voltage is at 10V, the device is still able to ostensibly produce 10V at its ouput when delivering over an amp (6 ohm load). I suspect this device will be good enough to feed your camera system but I can't absolutely say because you haven't specified current. There are other higher power devices that would fit the bill.
Can I wire a load to my battery if it is connected in parallel with the charger?
If the battery is lead/acid and the charging current is significantly more than what the camera load takes when attached to the above regulator then yes you can. If the battery isn't lead-acid then we need to know which technology it is.
How can I add a solar panel + controller to the previous circuit?
Playing safe, you can use a relay circuit that activates the relay when the AC power is applied to the charger - the relay contact can switch the battery from solar charger to AC charger in a few milli seconds. Playing a little bit unsafe, it's likely that your solar charger will have a diode in its output that protects the battery from discharge when the sun doesn't shine.
This very same component probably can mean that you can connect the AC charger permanently to the battery (and solar charger) BUT, you may need to add a series diode\$^1\$ in the AC charger's output when AC is off and the solar charger is feeding juice to the battery; the AC charger's output circuits may be activated by the solar charger and it's difficult to say what will happen - worst case it might pop the output transistor in the AC charger - best case no problem.
However, the chances are likely that your AC charger (just like your solar charger) will be protected from reverse voltages when power is down (or sun is not shining). You need to check this.
\$^1\$The diode needs to be a low volt drop schottky type capable of taking the charge current (again, you haven't specified max charging current so it's impossible to say but there are plenty rated for 10A and 20A continuous usage).
It's not impossible, it's just more complicated and expensive. Everything in your house is designed to run from AC. Many smaller products do take DC in but they come with an AC adapter because that's the only available source of continuous, inexpensive power nearly everywhere. The voltage required can be different for each device. The closest thing to a standard for DC power is probably USB 5.0V, which only offers enough current for small gadgets and not anything larger.
The way a solar powered house works is roughly: solar panel to battery charger to battery, to DC-AC inverter to wall outlets, plus another power regulator & meter if feeding extra energy back to the grid, which isn't a requirement. One could power a house directly with unregulated DC from the battery if the appliances were designed to run from it, but most aren't. If the battery voltage had to be regulated before distribution to the house, all you'd really be doing is swapping the DC-AC inverter for a DC-DC regulator, basically a different box with similar cost.
Due to the small size of the market for DC appliances (at the moment), they'd be harder to find and possibly more expensive than AC units. If a time comes when nearly every house has solar on the roof, they might be just as easy to purchase and maintain.
As to reusing wiring, a wire is just a strip of copper and doesn't care whether you put AC or DC on it, IF you stay within its capabilities. If you had to put a lot more current through the wire due to lower voltage, you might need thicker wires, different safety features in the wiring boxes, higher rated fuses and so on. You'd want different plugs on the outlets so nobody made a mistake of plugging an AC device into an outlet providing DC.
Overall, it's cheaper and simpler to put a DC-AC inverter at the battery than it is to gut the entire electrical system of the house and rebuild it, plus buying all new appliances, plus still needing small DC-AC inverter in each room for the devices which can't be repurchased to run from DC - which at the moment is nearly every gadget. You might think of the AC inverter as providing "backward compatibility" with the previous hundred years of electrical devices.
Best Answer
I'm not positive from your question, but I think you might be over-engineering a typical solar setup. All you have to do is connect the solar panels through your charge controller to the batteries. Then run your DC equipment straight off the batteries. The DC equipment will draw from the batteries as priority, but as soon as they start to drop in voltage, the charge controller will kick on and allow the solar panels to charge them -- which means that you will effectively be pulling the net power from the solar panels so long as their is power available from them.
The only reason you might try to skip the batteries is if you wanted to minimize the load you put on them, but honestly, you should just get a good quality desulfator to put on your batteries and then you'll never have a problem with keeping them in the equation.
As a side note, what a lot of people don't realize is that your solar panels could supply 36 volts to charge the batteries and the batteries will automatically act as a "soak" for the power, which results in 24 volts (if that's what your bank is rated at) available to any appliances you connect. Some engineers argue with this because its not mathematical enough for their minds, but in practice, it works perfectly. Real practice > the model.
To further back my point, most commercial setups will claim you need diodes in your system to prevent reverse current leaking back through your panels when they aren't doing anything. Try measuring the reverse leak current and you will laugh your butt off. It's tiny. The diodes are overkill in a lot of systems. Keep it simple.