The problem you are going to continually run into in what you describe is that the sound source position of the IR determines the localization of the IR. The relationship between the impulse (speaker, acetylene balloon, clapper) and the ambisonic microphone is fixed when you record that IR. You can rotate it but you can't really move the speaker around after the fact so to speak. You could potentially use the information learned from the IR to synthesize the properties of the room and create a reverb model (using something like CATT) that would allow you to freely position sources in that room. It's an interesting question, is there an IR reverb that models in that fashion. Some of the shoebox options (like the one in IEM) offer something like this? Dear VR Pro allows for free position within certain hard-coded (not user created) spaces.
Easiest would be to get a good ambisonic IR of the space and then tweak that to simulate your different positions, or just make several IRs with the source in the likely places you'll want to put your instruments.
As an aside, I'm loving the melda MconvolutionEZ for its simplicity and non-crashiness (for me anyways) in comparison to xvolvler.
I use
gratisvolver to create my impulses as I am not on a Mac.