Is Mangirl based off a real story to some degree?
It's pretty difficult to find information about Earth Star Entertainment, but from what I can tell, I don't see any similarities. According to its website, it was established in 2006 and its current director is Masashi Kikuta and the president is K. Makuuchi. Masashi is a man's name, so presumably, Masashi Kikuta is a man. The major affiliated talent for the anime is a woman, but there doesn't seem to be any specific link to anything that you mentioned.