Hi Jules, thanks for the question. (Early as it may be...)

having 2 insert effects/ part is one of the strong parts of the Montage...

We agree, particularly with Insertion Effects of this caliber.

Could i use the effects of the unused parts, by rerouting the audio out of a part and use it as the audio in to the effects of another unused part?

The short answer is NO, of course not, because INSERTION EFFECTS are apart of the VOICE architecture. You could no more do that than.. say I have a bass with four strings, the guitar play is only using four of his strings on a particular chord, can the bass borrow the two unused strings? (Perhaps a bad example, but just as impossible).

The longer answer is you always could - but it depends on your end goals (which you have not specified).
You can apply INSERTION EFFECTS on any PART be it internal or the A/D INPUT or a digital input Part external (routed into the synth).

So when doing music production the Motif ES/XS/XF handled this with the audio expansion (mLAN16e, mLAN16e2, FW16E). You can route external signal back through the internal Effect processors and record it to a discreet assignable output. In effect, pun intended, using the Motif ES/XS/XF as a processor in your studio.

With the Montage you have 32 audio outputs (stereo + 30 assignable outputs) via USB, built-in. The 30 assignable outputs can be configured as mono or in odd/even stereo pairs, as necessary. So any signal you wished to process additionally could be routed through the Montage, processed and routed back out on a discreet port. This is all theoretical because we await the finished Montage. But in theory, you will be able to operate in this fashion.

Even at higher resolutions (up to 192kHz) you still will have options about using the Montage to process external signals... and that is just the beginning.
We'll have more details as time goes forward.