We generally try to keep it to one question per thread. Otherwise it can get messy. However, these both seem simple, so I’ll give it a go. I reserve the right to ask you to create a new thread if there are a lot of followups, though.
Ever user is in sonar_users automatically & that group doesn’t have to exist in your IDP, so it seems that’s your “normal users” group. Now you just need to make sure your “admin” group exists on the SonarQube side. Just create a new SonarQube group with exactly the same name (capitalization &etc) and you should be good to go once you grant that group the proper admin privs.
You can’t disable that. That’s how you get to SonarQube’s native auth to use local, technical accounts.