Capacitor polarity and overvoltage
i would recommend repeating the experiments with fresh (unmolested) caps each time, applying only 1 stress at a time: (a) over-voltage, (b) reverse polarity, what happens when you (a) first then (b), then vice-versa.  in your first experiment, the over-voltage may well have been the killer, but reverse-voltage on an electrolytic cap = instant heat death.

there is no perfect conditions to kill a cap, it depends on the type of cap, how long and how high the over-voltage is, what temp it reaches due to the abuse to further damage it, etc.

&amp; as Meletis said, if you really want to see a bang, try some tantalum caps!

but PLEASE, wear protective eye cover when doing these tests, caps WILL explode violently and spray toxic chemicals, &amp;/or molten metals, sometimes at high speed.  this experimenting is not worth an eye.  and do so with fire extinguishing apparatus handy too (fire blanket, extinguisher (of type apropriate to electric fires)).

your final question, not sure what you're really asking - but polarity is fundamental to electronics.