Where do I file complaints about returns and customer service?

One way is to send a message to Intuit account on Facebook.   You can also write a letter and mail to corporate headquarters  2700 Coast Avenue, Mountain View,, CA

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

https://twitter.com/TeamTurboTax

or

https://www.facebook.com/turbotax