TurboTax helped me file a tax amendment, but made several mistakes. I now have to repeat this process. Is it possible to escalate the issue to a Manager at TurboTax?

Contact TurboTax support and speak directly with a TurboTax support agent concerning this situation.

See this TurboTax support FAQ for a contact link and hours of operation -https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
They are available M-F  from 5 a.m. to 5 p.m. Pacific time.