Could you do a profile lookup and see if there is any error reason showing up in the "message.feedback" event ?  Also, following link has troubleshooting queries that can be run for further debugginghttps://experienceleague.adobe.com/en/docs/journey-optimizer/using/data-management/datasets/datasets-...

Kindly log a support ticket, if the issue is still open.

Right - csv upload won't trigger the event journey. Data matching the event criteria needs to be streamed in for the journey to be triggered.

Kindly take a look at the following note. Unitary journeys (event based) are triggered when the event criteria is satisfied and when the event is streamed into AEP. https://experienceleague.adobe.com/en/docs/journey-optimizer/using/configure-journeys/events-journeys/about-events  

If all the data required for personalization (in the message) is coming in from the event payload, then it is not needed.

No, that won't be feasible as "AJO Message Feedback Event Dataset" is a system dataset. You can potentially explore https://experienceleague.adobe.com/en/docs/journey-optimizer/using/orchestrate-journeys/journey-use-cases/technical-use-cases/custom-action-aep for collecting data in the custom schema...

Can you check the http status code by querying the following attributes from journey_step_events dataset for the respective journey ?  _experience.journeyOrchestration.stepEvents.actionExecutionError AS ACTION_ERROR, _experience.journeyOrchestration.stepEvents.actionExecutionErrorCode AS AC...

JOJourneyVersionsDs is an internal Dataset and not publicly available.

Is this happening for all the fragments or only this specific fragment ? If it is the latter, kindly try reconfiguring it and if it still shows the same error, kindly log a support ticket with client care team.

You can query the profile snapshot system dataset to find out the qualified profiles against your audience.SELECT count() FROM profile_snapshot_export_ID -- replace export id WHERE segmentMembership['ups']['segment ID'] IS NOT NULL AND segmentMembership['ups']['segment ID']['status'] IN ('realized')...