This is clearly false- just consider any currency that has experienced hyperinflation. Currencies backed by commodities are not subject to such troubles.

This is completely false. When silver was discovered in Nevada the price was massively affected. Hell, Spain's economy was wrecked by the importation of gold from the Americas.

If we ever figure out how to bring asteroid resources to earth reliably, or if someone discovers a large deposit of gold you will see just how "stable" the value really is.