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.
An update from the future, as I've been running into the same problem, and found this post written by someone who sounds REALLY smart ;)
In mendix 10, when I ran into this issue, the fix was almost the same, except that I needed to clean the vendorlib as well as the userlib. Any files in the vendorlib, I removed from the userlib (keeping them separate until I was sure everything worked).
I'd also highly recommend creating an import template from a test document. I wasted A LOT of time trying to do it by hand again, but there's no need if you have a nice document with test data to play with.