It appears that I was mistaken about the way that the Inbox connects to LU email. I thought that messages would be copied to LU email, but this is not the case. Inbox messages/conversations are not copied directly to email.
While Inbox messages are not copied to email it is possible to set an immediate notification which will be sent to your email. The notification should contain the contents of the message. It is possible to reply to Canvas from the email notification.
I tested this and was able to reply to the notification message from email and have it posted back to the student Inbox. There is some lag when replying this way. The response is much faster through Inbox. I contacted Canvas support and they mentioned that a reply from email may lag up to as much as an hour depending on the server activity. If a message is time-sensitive I recommend replying from the Canvas site.