Anyone can use Power BI REST API | Access Power BI REST API without Azure App Client ID 

Yes. We have api support which you can from PowerShell - learn.microsoft.com/en-us/rest/api/power-bi/reports/update-datasources-in-group

Yes. Try it option got removed and it will be added again soon but not sure about the timeline. You need to use any other rest api application like powershell or postman

Try it option is disabled by Microsoft now. It will be added soon. The other option would be using Postman tool or Powershell

You can refer the following link but you can't except same option here as it is in onprem - app.swaggerhub.com/apis/microsoft-rs/PBIRS/2.0

Microsoft removed try it feature recently.

Try it available now. Pls check