Refactor backend to properly get user models and query models

## Basic Information
**[Give a brief summary of what the issue is about]**
Currently it is inconsistent across the apps on how the user models are being acquired and how models are being queried and consumed.

E.g. instead of using filters to select objects, use `get_model_or_404`

ensure all `get_user_model` is used everywhere

## Other Information
**[Are there any additional information that we should know about?]**

Start by locating the backend apps that acquire user models and query models, including uses of get_user_model, filters, and get_model_or_404. Review how each result is consumed; done means the apps consistently use the intended model-acquisition and query patterns.

Written by the indexing model from the issue text.