Is there any way to copy the entire best answer, along with all the TeX codes at once (something like right click => select all => copy) for reference or further offline use?
Another way is via the Revisions page. If an answer has been edited, you can click the 'Edited by' link underneath the answer; otherwise, you'll have to construct the URL manually. It contains the answer's ID and looks like this:
You can click the [source] link to see the source of the post. This works even if the  link is disabled (e.g. because there is another suggested edit pending).