Yes, as this is the least harmful default option after you have first opened the dialogue.
How is MP3tag to know which field you want to edit/delete or create a new one? You have help MP3tag a little and ...
select the field that you want to edit.
press the "Edit button"
enter the modifications in the subsequently opened dialogue
finish the editing with OK
press OK to apply the changes.
As for the "functions" that you do not know how to create: first of all it is "action" - you will find a separate menu for this.
Secondly, there is a guide in this forum in the FAQs how to create actions:
/t/967/1
Perhaps this initial help plus the advice on the "format value" action may save you from merging the already filles COMMENT_ADD fields with COMMENT manually.