Weird that -DuseExternalMongoDb does not work, I will check in the weekend that we didn't break anything.
Did you also set the environment variables? i think there is a check that only uses the external database if MONGODB_HOSTNAME is set.

As an alternative, you can use the class MongoDBBackendTckHelper and with @ClassnameFilters select the test you want to run. Tests from the ide will not sue the emebedded datastore (if you don't run them using the maven build).

Also, have you tried with one of the older versions for the embedded datastore? Maybe they haven't updated the plugin with the latest version.

If you have something ready, you can also send a pull request with the tag [PREVIEW], this way we know that it's still a work in progress but we can start giving you some help. I'm not sure right now about the problem with JTA but I might have a better idea when I see some code.

Thanks for taking this!