Isn't it much easier because you have capacity (weight carrying ability) for many more cells, and the chances too many of them fail is much lower? If you have 3 cells, 1 of them can fail at any given time, and you're left with 66% of power, but if you have 200 cells, and 1 or 2 of them fail, you don't even notice. How do you mean that it's harder? Electric cars certainly don't seem to have any problems with this and their batteries are much more reliable than the ones available for drone usage.
Also with big size/capacity you can easily include additional systems like temperature control which increase reliability because they don't weigh as much in relationship to the whole vechicle.
Don't think space (not the biggest issue here as you say), think weight, power and the ratio.
If a helicopter is doing it with a single main rotor, I'd be surprised if it can't work with a hexacopter and redundancy.
I'd be more worried about noise. A consumer drone is noisy enough, though mostly just noticeable because it's a new thing people are yet to get used to.
Helicopters auto-rotation is not at all comparable to a drone with one or more failed rotors, a helicopter that is autorotating is comparable to a gyrocopter, it will fly relatively well, a drone with one or more failed rotors will not fly well or even at all.
A parachute system only helps if you are high enough. Looks like the flying thingys we are talking about here will be close to obstacles a lot and not very high up much of the time given how they are to be used.
Battery redundancy: The scale argument again. This is an order of magnitude harder to achieve on a much larger man-carrying "drone".