control V does not work to paste in email headers

You need to go to Keyboard Shortcuts in System Settings. If you haven't created your own or modifie them then try resetting defaults first. If that doesn't work then you'll have to create a new one specifically for the Mail app.

You have to reference the Menu Elements. Mail-specific shortcut will look like

Mail->Edit->Paste

then add the keys cmd-v

Only thing I can think of....

Instead of Cmd-v when trying to paste go to the menu bar Edit drop down and check that CMD-V is displayed next to the "Paste" option. If it's not it means that the keyboard shortcut for Paste has been disabled for the Mail app. You'll have to go to System Settings > Keyboard and edit the shortcut to make it apply to Mail.

I had the same problem with the paste option not being shown in the menu bar. For me it was because I'd previously assigned a keyboard shortcut that uses CMD-V which was clashing with Mail. I removed that command and the paste issue was fixed and I can now paste into the email headers.

Click on the email you want to copy the entire thread. click on view and organize conversation and click on expand all conversations-- at that point you just take your mouse and drag it over the conversations when you finish dragging over the whole thread copy now you're ready to paste it onto a document and you go to work or pages or whatever document you want and you just pasted into the document now you have all the emails you want saved in the document name your document. I hope this helps Mary

You're welcome. Have good and pasty weekend!

It just worked on my 2020 iMac running Ventura 13.5.2.

Your OS installation may be flawed.