Mixed use or leaving the battery in the appliance can cause battery leakage.

Whereas some heat is normal, unusually hot batteries could be caused by over-discharging or short-circuiting. In such case, remove them from the device immediately.

Battery performance greatly depends on the device and on the conditions.

In general, appliances are designed to match the characteristics of dry batteries and to work within the voltage range of 1.5V to 0.9V. Thanks to this characteristic, Ni-MH rechargeable batteries can be used in appliances which require alkaline batteries, even with lower initial voltage. 

Turn off the device and remove the battery immediately. Remove the remaining leakage using a cotton swab. Avoid skin contact at all times.

Yes, the batteries' weight does change during usage.

Never mix batteries. Mixing batteries causes an imbalance in the energy flow. Some batteries will overcompensate this by discharging faster than usual - greatly reducing their lifespan.

Change the batteries as soon as the device starts malfunctioning. Remove the batteries if you plan on not using the device for a longer period of time.

At this point Panasonic does not produce AAAA or LR61 batteries.

You can, but they’ll perform best in medium- to high-drain devices.