1. The loan payment due date is likely information stored in the Core so you could make a core call with the SDK to retrieve that. It may also be in a HADE, depends on how your env is set up.
2. Depending on how one goes about this (in order to prevent overloading the Q2 system and/or the Core) you could use the SDK to write a periodic job that runs once a day maybe and sends out emails to users.
3. I will let someone else answer this.

Application outlets would be the recommended way to provide an alert and redirection post login. This is possible with UUX Payload as well. You have the ability to conditionally open a modal using Tecton's showModal capability and then redirect to the loan payment form using the navigateTo capability.

https://docs.q2developer.com/guides/frontend_development/application_level_outlets.html https://docs.q2developer.com/guides/frontend_development/uux_payload.html
https://tecton.q2developer.com/capabilities/showModal/
https://tecton.q2developer.com/capabilities/navigateTo/