From the info threads at Lunar, this is written by the scenario writer for one of the Kanon's (the games I think). He left Key after finishing Kanon for something that was never published.

sola is not based off any game that I can find, h-game or otherwise.

Character designer from Da Capo and Canvas2 (sola's characters look a lot like the ones from Canvas2). Director is from Utawarerumono.