How to implement custom field mapping and validation in a Django REST Framework ModelSerializer?

Question:

I'm implementing an EpisodeSerializer using Django REST Framework and need help handling several serializer fields and validation rules.

My serializer should include the following fields:

id: Read-only.
title: Required, readable and writable.
host: Accepts the host ID as input and returns its ID as output.
host_name: Read-only, retrieves the host's name using source="host.name".
duration_seconds: Required, must be at least 1.
level: Must be one of beginner, intermediate, or advanced.
token: Write-only input field mapped to the model's secret_token field.
is_long_title: Read-only computed field using SerializerMethodField. Returns true if the title length is greater than 15.
created_at: Read-only and automatically generated.

I also have two custom validation requirements:

1. Title validation

A title containing only whitespace characters should be rejected with exactly this error message:

Title cannot be blank.

2. Empty PUT/PATCH requests

If a PUT or PATCH request is sent with an empty request body, the API should return HTTP 400 with the following error under the non_field_errors key:

At least one field must be provided.

My questions:

What is the recommended way to map an input field named token to a model field named secret_token without exposing it in the response?
Should I use validate_title() or override another serializer method for whitespace validation?
What is the correct approach to implement is_long_title using SerializerMethodField?
How can I handle empty PUT and PATCH requests consistently and return a custom error under non_field_errors?
Should these validations be implemented inside the serializer or the view?

I'd like to follow Django REST Framework best practices rather than putting unnecessary logic in the view.