This means I could accidentally open a mail with a virus or other as I can no longer view it unopened.
If that is your concern it is not possible for an email message to affect a Mac in any way, even if you were to open the email message, and even if you load any remote content. You can prevent the latter in its Preferences > Viewing.
As for "Classic View" it has been eliminated from Mail, but if you are not seeing a three column presentation then explain what's missing. By default the leftmost column contains the Mailbox List, the center column is the Side Preview, and the rightmost column contains the body of the selected message.
I surmise you want to show the Side Preview, and it is not appearing. There is a longstanding bug in Mail that prevents that column from appearing if you de-select it and subsequently re-select it. To restore that column follow dialabrain's instructions.