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/

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.