Magnesium will do these things and is very flammable as you say, however these are not the properties of it's alloys. Most of its alloys are alloyed with aluminium anyway, increasing its strength whilst keeping its low density. It's alloys are perfectly safe to store as normal. It is highly unlikely that the average person will come into contact with pure magnesium scrap as it will come with a highly flammable hazard symbol and isn't used to make items on it's own. If you do it is most likely you will see it in flares as it is used in powder form in the signal flare cartridges. Aluminium will react with silver nitrate though how long it takes depends on how thick it's oxide layer is.