Without bacteria there is no complex life, thus no us, so therefor no economy. Also, bacteria can be engineered to produce various substances which can be used in medicine. Such as human insulin

Yes, all substances have a unique melting points and freezing points, some will melt far faster, or with less energy, than others as the particles which make up the solid form are held together by different levels of intermolecular force. The speed at which the solid liquefies is relative to the temperature applied though.