Section 1

1. Yes. Open registration is the least restrictive sign-up configuration where the portal allows a user account to be registered by providing a user identity. Refer to https://docs.microsoft.com/powerapps/maker/portals/configure/configure-portal-authentication.
2. Yes. The user can redeem the invitation code when signing into the portal. Refer to https://docs.microsoft.com/powerapps/maker/portals/configure/set-authentication-identity.
3. No. Guest users in Activity Directory are not enabled to access the portal.