Hi there @brsastre 
So to try answer some of your questions 
- Does data transfer only works with Strapi Cloud, or it would work on self hosted plans, such as AWS?
No DEITS works with self hosted and cloud covered in our Data Transfer Documentation 
- What are the main benefits of Strapi Cloud over AWS self hosted services?
Strapi Cloud has all you need for production ready to go, you push to github and from there hit deploy. It includes a database (Postgres for now) CDN, Email Provider more to come
In simple terms “All “batteries” included but swappable. No lock in”
Check out Strapi Cloud 
- Is it possible to transfer everything from a local environment to a production environment with a few clicks? by “everything” I mean code and database.
So with DEITS you can transfer data and media, while deployment you can deploy your code.
You develop locally setup your schemas and endpoints push to github and deploy it.
What are “Seats” ?
Please check out the Docs here on Usage & Billing 
As it’s explained there
“Seats represent the maximum number of users that can access the Strapi Admin Panel. Each plan comes with a default number of seats.”
When it comes to sendgrid the best way to look would be to look at the cloud pricing as it shows the Emails. Cloud Pricing 
Hope this answers your questions 