I've knit the different features of the model into one and tried to mirror it over the right plane but keep getting the "one of more features is no longer there" error.
It worked fine for me. Make sure you choose "Bodies to Mirror" instead of the default "Features to Mirror".
wow. That worked. Thank You!!
What if you mirror Bodies rather than Features?
I have made the assumption you want to mirror about the right plane. Without doubt, with surface models, you are far better off mirroring bodies rather than features- firstly it is far fewer clicks, and secondly far more reliable. When mirroring features, SW has to rebuild all those features in different positions to where they first were made making the chance of failure very high. This is the way I mirrored it and got the below result:
Retrieving data ...