Wait, I'm confused about what you are saying you are doing to start the forwarding process. To forward an email thread, you click on the little right-pointing arrow that appears on the line above the text in the preview window, if you use preview, or just open the thread you want to send and the right-pointing arrow will appear on the line just above the text. You move the pointer to the line and the arrows appear. One icon is the trashcan to delete the message, one is a single arrow to the left to reply only to the sender, one is a double arrow to the left to replay to all and the final is the right pointing arrow for forwarding. You can also choose "Message" in the top bar and then "Forward." No select all, no copying, no pasting. The thread opens in a new draft message with the forwarded material marked on the side by a line. Now you can edit to your heart's delight on that material and only what you select gets deleted.