For every environment you have config interface available there you config the values  https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/implementing/using-cloud-manager/environment-variables.html?lang=en  https://experienceleague.adobe.com/docs/experience-manager-clou...

set the redirect path while calling your SAML auth handler.

Yes you can using sling superResourcetype , the only issue is if there is any upgrade happen in acs-commons version in future then the referred field will also get impacted. 

Check ACLs whether you have write permission or not.

Check from developer console .Ref : https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/implementing/developing/development-guidelines.html?lang=en  

You can still use it but only thing is for every change you need to push the code and deploy in dev as well .As you cannot edit /apps in AEM as a cloud directly from crxde.https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/release-notes/aem-cloud-changes.html?lang=en 

After stage testing step is it keep going till deploy to production? 

Can you share the snapshot , as approval is required for production deployment.