Actually, in the Summery and even in the first chapter of the Darkfic, it's implied that the parents are responsible for the puppeteer. If you read or listened to the reading from Scribble than you saw/heard that the parents already knew about the cutie mark and they weren't surprised or amazed in anyway, or even glad that she got the cutie mark. These are the lines from the chapter that I'm sure caught a lot of attention “Just because you buy her some special cello doesn’t mean she has any talent,” “And I will not keep wasting time on her! It is done, she has the cello and a mark,”. In the summery was also mentioned that the parents played with dark magic. What I think happened (and is kinda true) that her parents din't want Octavia to have a cutie mark from something else unless it was involved with music. So her father bought her "a cursed cello" and also put a spell on Octavia which gave her a cutie mark. But of course neither him or the mother knew the side effects of the spell, and they both paid the price. I don't think puppeteer even existed, like Octavia said "It's not some unseen force, they're obeying me!" it was like making a deal with a devil. The strings helped Octavia get her freedom, but because deal is a deal she had become their puppet in return.

