User Acceptance Testing (UAT) is the best method, as it ensures appropriate users can access the form and verifies that the data update process works as intended in a real-world scenario.

"Tasks attributed to user" is the correct context type, as it allows you to display pending tasks assigned to each individual in the application.

A portal interacting with records can capture public outage reports, contact details, and upload supporting documents to a defined folder, making the process accessible to the public without requiring authentication.

While Appian and the integration technically support 1 GB file uploads, handling such large files during interactive sessions can negatively impact application performance, user experience, and resource utilization.
A better approach is to request that the business allow these files to be uploaded asynchronously (e.g., overnight batch process), by temporarily storing them (e.g., in a staging table or folder) and processing them when system load is lower. This makes the process more performant and reliable without straining Appian or the external system.

Process errors that remain unhandled will stall instances; those instances never reach an end event, driving the completion rate down.
User-input tasks without deadlines, exceptions, or escalations can sit indefinitely when users do not act, leaving instances in an active state and lowering the overall completion percentage.