Who's actually purple Reimu?
Simple, it's Reimu, but at the end of MS, Alice opened her grimoire and turn Makai into ash and the remains are found on UFO.
Reimu died, Mima disappeared because she used her last power to protect Marisa because Marisa is still alive, Reimu's reincarnation AKA the black Reimu still remembers Marisa.
The Hakurei Reincarnation works like this: You die, if your close friend still alive, your memory is passed through your next generation.
The hakureis lived for a very long time and never actually interact with others, the first on to do this is the purple Reimu, being the best friend of Marisa, so even if She died, as long as Marisa still alive, her memory still remains.
Well, it's A very long explanation of what happened at the end of MS (BTW Yuuka is invincible in her ball of light shape, so she remains docile for60 years as a ball of light until PoFV)
Sorry guys, Mima will not be going to be in another Touhou game :'(