These are wired parallel, so it's supplying the same voltage as two batteries, but will only last half as long before dying. It wouldn't turn on at half voltage in series.
Still gonna have mild power problems though cause the battery isn't seated well and will possibly short out when you move the mouse.
If the battles are wired in series (think train cars) it increases the voltage of The circuit. Which means if the electronics require the higher voltage this may not work well, if at all.
If the batteries are wired in parallel then this is basically half a fuel tank.
In this case, we can tell it's likely parallel because the springs (negative terminals) are on the same side. When the batteries are in series they're usually placed in the compartment facing alternating directions.
Depends on the configuration, but it won't explode. It may just act as if it's on low battery for a while then stop working as if the battery is drained, when the one battery might only be half drained. If it's in parallel it could work but with about half the runtime, but it would do just the same fitting just one battery in the proper orientation.
From understanding my old GameBoy that had 4 AA batteries in alternating rotation, that had 6V (1.5V each battery). Chaining positive and negative together increased the voltage.
Since this has them pointing both up, it's just 1.5V but it's as if you put a half sized battery.
Basically, the same, just less amperage because of a smaller battery (if compared to 2 of the same).