Yes, if you want to use your tracks in MIR you will need to use the audio input plugs. However, you may find that some of the processing you are doing with your UAD cards will be unnecessary when you are using MIR. The other thing to remember is that MIR isn't a reverb. You can't just route stereo stems into it as you would a reverb plug. That is not what it is designed for. Therefore you will need to think carefully about workflow in order to find that best way to do things.
I would have thought that one good way would be to send each instrument that you want to route into MIR back to Cubase from your slaves via their own outputs, which will show up in the VSTi output section in your mixer. Then I would put an audio input plug on each of these and route these into a local instance of MIR Pro, obviously hosted in VE Pro. This is slightly convoluted, but you can set up a Preset to call up for future projects.
I would also recommend trying the demo version out before buying, just to make sure that the workflow is acceptable to you.
DG