Question about tickets/comments API

We have issue with data migration form some external system info freshdesk:

  1. When we create new ticket and comment in Freshdesk we would like to set created_at field to date when it was originally created in external system, however no matter what format we provide, we get error, so we had to remove this field. But it is important as client is migrating from another system to be able to set created_at to original date.
    So is there a way how we can set this data during initial migration from another system?

  2. For ticket comments API, how we can set body_text? We can set only html content? If we try to set this field we get error, so we had to remove it.

  3. For initial integration, is it possible to increase API limits temporary?