I think now you got it! 😊 That's the way to use MIR. - The maybe most-quoted sentence in this forum is "MIR is _not_ just another reverb." It is meant to be a virtual 3-dimensional room and should be treated as such.
Sidenote: A Decca-tree is just one of the many possibilities to set up a main microphone array for orchestral recording. The fact that is was propagandized by a well-known record company (Decca) might be one of the reasons for its popularity - but other setups are able to surpass the "tree" in several aspects.
It might be a good idea to take a look at the built-in MIRx-settings and/or all the "Venue Presets" available from your User Area to get a feeling for the typical use of MIR Pro.
Kind regards,
/Dietz - Vienna Symphonic Library