4 is not possible out of the box, here you have two options:
For both of them you will need custom field of string type on request e.g zcustcomment so you can access it in email template as @{call_req_id.zcustcomment}
1 option - create trigger that on research comment insert will run spell code that will populate zcustcomment field with the description.
2 option - you need to modify researc form to grab the content of description field and set it to custom field using hidden input
Personaly i prefer first option since it will not cause any problems during migration to next version