"The Audio Montage is a 32bit audio streamer. This means that it non-destructively upscales all audio used within it to 32bit, right?
Right
So, even though the audio used in the project is actually at 24bits, I would need to dither to 24bit if I wanted to render out at 24bit because of this upscale and subsequent truncation?
Yes, as far as any processing is performed (levelling, fades, plugins), which is the case 99% of the time. But the process of going from 24 to 32 bit is lossless: going back to 24 bit does not need dithering if there is no processing, ie. if the samples are not changed.
And I would also need to dither to 16bit even if the original mixes and masters were at 16bit?"
Yes
Philippe