Kindly take a look at this documentation https://experienceleague.adobe.com/en/docs/journey-optimizer/using/orchestrate-journeys/create-journey/testing-the-journey where in you can use test mode to validate the journeys (either audience qualification or read audience based) There is also a dry run f...

I would avoid using it as it is an internal dataset and not publicly available and can be changed any time breaking any integrations built on top of it. https://experienceleaguecommunities.adobe.com/t5/journey-optimizer-questions/query-the-journey-properties/td-p/710026 

Not in the current state (with respect to all the three attributes). Kindly log an enhancement request for this.

Journey status is currently not available

You can query journeys dataset and status, created by are not available in the current state (as you have already mentioned)

No, complex data types (array in your context) cannot be updated.

You can either put a condition like .....pathToPlanId != '' or remove the filter if you want to count all of them in ObjectLevel4 array object by specifying ...pathToObjectLevel4.all()}

You can validate it on the following lines listSize(#{ExperiencePlatform.ProfileFieldGroup.profile._ObjectLevel1.ObjectLevel2.ObjectLevel3.all(currentDataPackField.PathTofieldWithinObjectLevel3 == '<filter value>'). pathToObjectLevel4.all(currentDataPackField.PathTofieldWithinObjectLevel4 == '<filte...

Alerts are supported for two specific cases as mentioned in this documentation https://experienceleague.adobe.com/en/docs/journey-optimizer/using/reporting/alerts You can potentially leverage query service https://experienceleague.adobe.com/en/docs/experience-platform/query/clients/overview  and con...

which API end point are you using ?