I think it is possible to exit with an MPhil but different unis do it differently so you should really talk to your supervisor/graduate studies officer about how to go about doing it.
If you fail the MPhil the first time I think you have a second shot at it. If you fail the second time I don't think you get anything for it. Again, each uni has specific rules and regulations for this.
Losing interest is a reasonable reason for giving up on your PhD but being scared of the viva is something that can be dealt with if you want to continue. PM if you want to talk some more.
Your university and supervisor will be able to help you prepare for the MPhil viva.
If you drop out you drop out. There is no alternative. You can ask yours or other unis if you can transfer credits and do an MA/MSc/MRes (though an MRes will also probably have a viva at the end of it).