TL;DR:
Request 1
Is it possible that you can implement editing of all fields in the external window?
The idea is that each field is represented by a header, and the text under the header is the content of the field.
This will allow you to edit the letter in its entirety in a very pleasant way.
Example:

Request 2
On the other hand, I see that you use CodeMirror as editor. Is it possible that you can implement VIM mode?
TL;DR:
Request 1
Is it possible that you can implement editing of all fields in the external window?
The idea is that each field is represented by a header, and the text under the header is the content of the field.
This will allow you to edit the letter in its entirety in a very pleasant way.
Example:

Request 2
On the other hand, I see that you use CodeMirror as editor. Is it possible that you can implement VIM mode?