Explanation
The OAuth 2.0 user-agent flow and the OAuth 2.0 web server flow are both suitable for building a custom mobile app that can access Salesforce data without prompting the user to log in again1. Both of these flows use a refresh token that can be used to obtain a new access token when the previous one expires2. The user-agent flow uses the Canvas JavaScript SDK to obtain an OAuth token by using the login function in the SDK2. The web server flow redirects the user to the Salesforce OAuth authorization endpoint and then obtains an OAuth access token by making a POST request to the Salesforce OAuth token endpoint2. The mobile agent flow and the SAML assertion flow are not valid OAuth flows for Salesforce3.
References: OAuth Authorization Flows, Mastering Salesforce Canvas Apps, Access Data with API Integration

Explanation
The best approach to simplify the authentication process and reduce cost and maintenance is to configure UC1 as the Identity Provider to the other four Salesforce orgs and set up JIT user provisioning on all other orgs. This way, users can log in to any of the five orgs using their UC1 credentials, and their user accounts will be automatically created or updated in the other orgs based on the information from UC11. This eliminates the need to purchase a third-party Identity Provider or manually provision users in advance. The other options are not optimal for this requirement because:
Purchasing a third-party Identity Provider for all five Salesforce orgs would incur additional cost and maintenance, and would not leverage the existing user base in UC1.
Not setting up JIT user provisioning for other orgs would require manually creating or updating user accounts in each org, which would be time-consuming and error-prone. References: Salesforce as an Identity Provider, Identity Providers and Service Providers, Just-in-Time Provisioning for SAML

Explanation
The best option for UC to create the identities of its e-commerce users with the customer community is to use SAML JIT in the customer community to create users when a user tries to login to the community from the e-commerce site. SAML JIT (Just-in-Time) is a feature that allows Salesforce to create or update user accounts based on the information provided in a SAML assertion from an identity provider (IdP). This feature enables UC to avoid duplicating user registration on both applications and provide a seamless single sign-on (SSO) experience for its customers. The other options are not optimal for this scenario. Using the e-commerce REST API to create users when a user self-registers on the customer community would require the user to register twice, once on the e-commerce site and once on the customer community, which would degrade the customer experience. Using a nightly batch ETL job to sync users between the customer community and the e-commerce platform would introduce a delay in user creation and synchronization, which could cause errors or inconsistencies. Using the standard Salesforce API to create users in the community when a user is created in the e-commerce platform would require UC to write custom code and maintain API integration, which could increase complexity and cost. References: [Just-in-Time Provisioning for SAML], [Single Sign-On],
[SAML SSO Flows]

Explanation
A 2-Factor Authentication (2FA) solution is a type of multi-factor authentication (MFA) that requires users to provide two verification factors to access a system or application. The verification factors can be something the user knows (e.g., password), something the user has (e.g., phone), or something the user is (e.g., fingerprint). A 2FA solution can help prevent common cyberattacks that rely on stealing or guessing passwords, such as:
Key logging attacks: These are attacks where a malicious program records the keystrokes of a user, including their passwords, and sends them to the attacker. A 2FA solution can prevent this attack by requiring an additional factor that is not typed by the user, such as a verification code sent to their phone or a biometric scan.
Phishing attacks: These are attacks where an attacker sends a fake email or website that looks like it came from a trusted source, and tricks the user into providing their credentials or other sensitive information. A 2FA solution can prevent this attack by requiring an additional factor that is not known by the attacker, such as a verification code generated by an authenticator app or a hardware token.
Dictionary attacks: These are attacks where an attacker tries to guess a user's password by using a list of common or likely passwords, such as "password" or "123456". A 2FA solution can prevent this attack by requiring an additional factor that is not based on a password, such as a fingerprint scan or a facial recognition.
A man-in-the-middle attack is when an attacker intercepts and alters the communication between two parties, such as a user and a website. A 2-Factor Authentication solution can help prevent this type of attack by requiring a second factor of authentication that the attacker cannot access or spoof, such as a code sent to the user's phone or a hardware token References: 1: What Is Two-Factor Authentication (2FA)? | Microsoft Security 2: What type of attacks does Multi-Factor Authentication prevent?