How did the Earth Dragon Balls revive everyone killed by Frieza?
Um... they weren't limited to only 1 person at a time. 

It was the Namekian Dragon Balls that were limited to 1 at a time.  The Earth Dragon Balls had no such restriction. In fact, they were used before to revive multiple people that Piccolo killed at the end of the Piccolo saga

Further reading : Dragon Ball