Wouldn’t integrated fuses or breakers placed along the lines and between battery cells reduce the threat of electrocution?
No. The electric motor has to draw hundreds of amps at startup. A human touching 300 volts will only draw milliamps, due to the internal resistance of the human.
It's not the voltage that kills, it is amperage through the heart that kills, and that only requires about 100 milliamps. The amount of voltage required in order to get that 100 milliamps varies with how well you are connected to the circuit, how much resistance there is, how much body tissue is between the two contacts, and what you're made of.
Defibrillator paddles are placed right on the chest, coated with conductive grease (you've seen it in the movies, I'm sure) and they are designed to up the voltage until the desired amperage is obtained for the desired length.
If you grab the positive terminal with one hand, and the negative terminal with the other hand, I doubt that 300 volts would kill you, but I wouldn't try it.
On the other hand, I've had spark plug wires bite me many times, and that's 10,000 volts and up. Very low amperage, however. Same with those "novelty" packs of gum that give you a shock when you grab them. High voltage, low ability to deliver amperage.