I think just using some common sense is all you need to use for both questions.
Would you willingly put some Super Glue into your mouth? 'Nuff said.
Although there are those who stack batteries in mechanical mods, they do so at their own risk. There is a methodology to make it less dangerous, which I won't go into detail here to explain, but there are still risks regardless.
Some advanced processor-based mods are actually designed to use stacked batteries relatively safely, but they have built-in safety components that will automatically shut the device down if it senses a dangerous situation.
These little batteries pack quite a bit of power, and if not respected and used safely dangerous situations can occur. I've had a "protected" solo battery go into thermal runaway because the fire button got compressed for too long in a pair of pants that I was not wearing. The ends blew out and released heated gas so hot that it melted the protective skin on the battery. Below is the battery.
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