We have a mobile app built using React Native and we have an external Identity Provider.

Here is our login flow:
Mobile App connects with Salesforce (using salesforce SDK) through OAuth and Salesforce performs SAML authentication with our external Identity Provider. So, for every user account in Identity Provider we have a Salesforce User record. And mobile tracks the Salesforce session.

Our goal is to avoid using Salesforce User record and have Mobile App perform SSO OAuth with external Identity Provider.
We don't know how to integrate from Mobile App to Salesforce for retrieving data. We want to use a single user account to connect with Salesforce, but we do NOT want to store the credentials of this user in Mobile App.

Any thoughts?

I'm not able to proceed further with challenge#7 because of the following error:

Challenge Not yet complete... here's what's wrong: 
There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: LJZGFZNJ

I'm using a brand new trailhead playground specifically for this superbadge.

We have a Community that runs on a custom domain with a visualforce homepage. Now the homepage does not open anymore.

Without an account: ERR_TOO_MANY_REDIRECTS
With an account you get a Visualforce error page: "there has been an error during loading a visualforce page."

In the org there was an update on Volunteer management and UI Lightning flow was enabled.

What could be the cause here?

I'm not able to proceed further with challenge#7 because of the following error:

Challenge Not yet complete... here's what's wrong: 
There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: LJZGFZNJ

I'm using a brand new trailhead playground specifically for this superbadge.