MMR are similiar in structure to other mining companies funding...

MMR are similiar in structure to other mining companies funding then spinning off subsid's. The most likely scenario's for an Advent spin-off are:
1) Priority float offer to MMR shareholders ((typically not option holders).
2) In-Species distribution of Advent shares to MMR holders (depends upon how many shares you have).
3) MMR will keep the Advent shares on behalf of MMR holders to retain control.

The other option is they won't float Advent (i.e. they change their minds given the strong shareprice rise of MMR) and raise the funds via issuing MMR shares or divesting a percentage of the project for funding (a.k.a MEO style).

There aren't really any downside for MMR holders in any of these scenarios or options, it's more the magnitude of benefit...

DYOR