1: HP Lovecraft was very well read in a vast array of subjects, including history, fantasy, mythology, folklore, archaeology, astronomy, psychology, architecture, physics and geography. He was also familiar with Theosophy and the writings of Blavatsky, as well as Sclater’s land bridge theory of the lost continent of Lemuria. It’s not much of a stretch to figure he probably read a translation or two of the Emerald Tablets since history was one of his life long passions.
2: Which version of the Emerald Tablets are you referencing? If it is the “Emerald Tablet of Thoth the Atlantean” by Maurice Doreal, then it is complete fiction, just like Lovecraft’s stories.