I need to know how to give clients the ability to comment on cards and lists but not delete comments, attachments, cards or lists.
Is this possible?
That would be the Observers feature, available only to Premium and Enterprise workspaces. You can read more about it here: https://help.trello.com/article/732-adding-observers-to-boards
I can see you have one Premium workspace so you should be able to use this feature there.
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