The first question would be why haven't magi already discovered such things - is it possible to use Rego to refine ores to pure materials, or use Muto to transform away impurities?
The second one would be has this already been discovered elsewhere (note the claims for Persian alchemists to have refined arsenic before Albertus Magnus)?
If the metal has always been there, then this implies that creation is a vast mystery waiting for people to unlock it. If it hasn't, that implies more things are being added to creation - how is this done? Is there a special group of angels whose job is to add in the Creator's new designs?
Arsenic bearing compounds themselves will be well-known to magi as they form major red and yellow pigments used for illuminating manuscripts.
I don't think so, as the metals are associated with planets (as you pointed out), and the astrological ages are associated with signs.
The only mythology I can think of would be the Greek one of an original Golden Age, then a Silver Age, then a Bronze Age, then an Iron Age. If the world turned into the Arsenic Age somewhere during the medieval period, it would explain a lot about modern pollution.
There are the mythical ages of men from Grece- Gold, silver, bronze, heroic and iron. Heroic does not exist in all accounts. Whether the discovery of arsenic heralds the start of a new age or not- it certainly was not a popular theory at the time, or to my knowledge even proposed since the Greek ages had been dismissed with the rise of Christianity, with only bronze and iron surviving into modern analysis, though now it refers to metallurgical technology rather than the idea of being races of men made of different substances.
It is a harder and tougher form of bronze compared to straight tin bronze. For making weapons, it is superior in nearly every way. However it does have a drawback for Faeries, in that it almost always has some degree of iron content.
Because it is a very early bronze, going back far into prehistory and copper age cultures, which in the middle ages nobody knew about - but perhaps some Faeries e. g. from the domus de janas.
No need for this to be so in any specific ArM5 saga, though.