Let me first answer the two questions directly:
Is there any way to get aware of such a comment-deletion?
There is no functionality on the site to get alerted to comment-deletions. You have to check. Likely, somebody could write a script to check automatically, but that's a bit orthogonal.
Is there any way to reply a feedback comment such that the deletion of the feedback comment automatically leads to deleting the answer comment?
This is not possible. It seems like something one could propose as a feature-request. It would fit better on the Meta site of the SE network as it would be a global change. That said, I doubt it would get high priority as comments are not a priority of the site in general and also see below.
Having answered the questions let me recall the process as intended in its basic form:
Somebody leaves a comment pointing out a problem in the post.
Post owner edits the post.
Post owner flags the comment as "no longer needed." [This notifies moderators but nobody else.]
Moderator deletes the comment.
To note the post owner never wrote a comment in all this. Thus the problem you evoke goes away; there is no need to get your comment removed if you do not write it in the first place.
Of course the four steps above are as said, the most basic case. Maybe the initial comment was not clear and there is need for clarification and you reply. There are also cases where it makes sense to alert the person that commented specifically to the change, and we arrive in your scenario. I certainly do not want to say that you should not reply to comments at all. I just meant to point out that there is not always a need to reply to such comments.
In this case one should proceed as Arctic Char said. That is, the user commenting originally, should after having been called back by the comment of post owner, delete the comment that is now "no longer needed" (because it was addressed by the edit) and then also flag the comment that addressed them (so that it can be deleted by a moderator).
Note that in the basic process outlined above, at least in clear cases, step 2 and 3 could be done by anyone not just the post owner. Also for 3, it makes sense to flag also for other users. Several flags will auto-delete.