Minibuffer

Delete minibuffer contents

While in a Helm session, you can delete minibuffer all contents with C-k (helm-delete-minibuffer-contents).

When the customizable variable helm-delete-minibuffer-contents-from-point is non-nil, helm-delete-minibuffer-contents deletes from point to the end of the minibuffer.

Mark, copy, and kill in minibuffer

Helm does not provide a way to mark/copy/kill a region in the minibuffer. The zop-to-char package provides a way to do this.