Final update – got it working! A coworker sent me a very easy to use tool to clean my userlib (it wasn't working manually). Here's the link: https://github.com/cinaq/mendix-userlib-cleaner
After a good scrub and dust, the application wouldn't run, but when I checked the error log, it was as a result of missing java actions that were in the excel importer module. So I reinstalled it, ran the app and tested the import and IT WORKS! Hope this helps someone down the line!
Update: I've been looking at this with some senior mendix devs and my current running theory is that it's probably a duplicate jar file. We tried to replicate the error in a clean 9.6.2 project with excel importer 10.0.1 and it worked fine. But when my team mate tried this locally he also got the same error. I'll update again hopefully with better news shortly.