Coinbase are an awful exchange, and their support is no different. In the past, other users have had to wait weeks or months to get simple things resolved. The best you can do is keep your issue on their radar. Keep sending tickets. Try PMing some of their staff who are active on reddit:
https://www.reddit.com/user/coinbasesupport and
https://www.reddit.com/user/justin_coinbase. Make some Twitter posts about it. Usually they are only actually motivated to help customers recover locked funds or accounts if it looks like there will be community backlash - they don't actually care about helping you get your money back.
All exchanges have struggled with support in recent years as they have experienced growth their support teams cannot keep up with.
Raising the issue on social media is a good way to get the attention of support, and may elicit a quicker response. I don't think opening multiple tickets will be helpful.