The peel and stick or water apply type ones last a couple week, they don't hurt because you're basically applying a sticker to your skin. It's a decal. We put them on kids. Totally safe.
Henna or "black henna" is basically just paint applied to the skin. It's not FDA approved and coloured hennas have adulterants added that can cause pretty serious allergic reactions.
Those longer term ones that market themselves as lasting multiple months or longer are done with "shallow needles" or by cutting into the skin often use toxic dyes and can absolutely be permanent. Lots of people find that the tattoos don't go away, and just become an extra shitty looking permanent tattoo. "Ephemeral" tattoos and the like are a total grift.
Matter still decays over time and will at one point cease to exist. Newton's second law is not a steadfast physical rule, it is an assumption made under classical mechanics to simply streamline physics, as the amount of matter that disintegrates, is annihilated, or decays over a "small" period of time in a given system is so small, that is makes little sense to take it into account.
However, it is still decaying, and at one point all matter will have decayed away or be annihilated.
Not really, insofar as temporariness is counterposed to permanence. This view comes from a misunderstanding of the concept of permanence: what is permanent is not necessarily eternal. Permanent is more like enduring, in the sense of lasting indefinitely, until further notice, or until longer than one could reasonably be expected to need it.
When we compare a tent to permanent housing, for example, we are not comparing it either to a structure which will stand for thousands of years or a home from which the occupant will not be allowed to move away for their lifetime.
Same thing with 'permanent' vs. 'dry erase' markers and so on. Tattoos are only temporary in the completely trivial sense that everything is.