In practice, it is impossible to answer a question by posting a comment. 600 characters is not enough space to give a fully reasoned answer.
Thus the rule about not answering a question in a comment is a pointless one. There is not the space to do so.
But if it is desired to better enforce that rule, which in its current form is pointless, the obvious solution would be to reduce the limit from 600 characters to some lesser amount.
If the legitimate purpose of a comment is only to ask for clarification of the original question, this might well involve asking a question: such as "have you inadvertently overlooked fact x?" or (if the question is ambiguous) "which of the two possible meanings, x or y, did you intend?"
The function of the moderators is to decide whether something posted (whether a comment or an answer) should be deleted. A user should merely flag a matter for the moderators' attention.
Ordinary users should not be invited or encouraged to conduct disputes on a user-to-user basis. That is the basis for flame wars, but the purpose of having moderators is to avoid such undesirable activity.
An ordinary user should ignore a request to delete, unless it comes from a moderator, and should certainly resist the temptation to argue with other users. Part of the job of the moderators should be to make it clear to all users that, because of the all too frequent consequences (such as flame wars), users should not invite others to delete.
If the site is to be conducted on an amicable basis, o/p's should not be making this kind of request. No one, except a moderator, should be dealing with issues relating to deletion of content, because any other arrangement merely invites trouble, and creates a situation which can all too easily get out of hand.