Hi Stephen,
I understand what you are concerned about. Seems like the Office 365 email service is suitable for you. As for questions about moving back, I'd like to explain it is not difficult for you. Just remove the custom domain from Office 365, delete the corresponding DNS records, then add your previous email service DNS records on your hosting provider. After that, your email service will work again.
Looking at your second question, after moving to Office 365, all emails will be routed by Office 365, it will make no difference whether the users are no the trial or not.
Meanwhile, if you have any questions, please feel free to let us know.
Thanks,
Nate Cui