Recently I got invitation to use Google's new tool Google Optimize. I have been trying to add it using Adobe's DTM using Google Universal Analytics tool and adding the container tag using "Customize Page Code" option. The Optimize libraries are being called, however I am seeing following error in the page console itself:
I'm not familiar with Google Optimize, but it seems like the error you were receiving is a pretty common one. A quick web search of "MIME type ('text/html') is not executable, and strict MIME type checking is enabled" brought back several links from Stack Overflow and Get Hub with similar issues. From the threads I read, it looks like it may be a configuration issue with the script and the web server.
Have you tried deploying the Google library outside of DTM on a test site? If you still receive the error when deploying the code outside of DTM, we know that the error must be coming from the library itself.