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Following up on (1), it is typical for the negative (black) terminals to be connected together, and to the boat, as a "common ground." This will still allow separation of the battery power because the positive (red) terminals are usually kept separate by the battery selector switch. The potential issue with your setup is that anything that goes wrong with one battery (failed cell resulting in low voltage, device left on, overcharging, whatever) will bring the other battery along with it.
Photo (2), the brass T-valve is by my estimation most likely attached to plumbing intended to expedite the use of an oil changing device - some kind of vacuum pump. A nice feature. The red handled valve is a seacock for either the engine raw water intake, or (more likely by my guess) for a cockpit drain. You'll need to follow the hose.
Photo (3), I agree with Cajun, that's a bit of a mystery! The bronze canister part is a raw water strainer (you can see one side of it labeled IN). The two valves are stem valves and are not suited for marine use, although tens of thousands of them were installed in the '60s and '70s, even in brand new boats. The ball valve, like in photo 2, is the right way to go. It is likely that those are different lines. But if they happen to be the same line, there might be a fitting between those two valves allowing winterizing antifreeze to be poured into the raw water line into the engine without accidentally leaking the other direction and out of the boat (the valve closest to the water supply would be closed; the other valve, open).
Photo (4), my money is on this being the engine raw water intake valve.
Photo (5) is a nice spin-on fuel filter. Replace this filter every few years. I always write the date of installation in a few prominent places on the filter, although when you take it off, fuel might drip on the writing and obliterate it. Note, they're called "spin-on" filters and not "spin-off" filters. They can be super difficult to remove. Even with a strap wrench, which in my engine room is hard to put on and hard to activate, I always seem to end up mangling mine no matter how conscientious I was when putting the new one on. We often suggest installing a small in-line "polishing filter" between the fuel pump and the carburetor as extra insurance.
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