The official OS is RO4 Select/Adjust.. I presume RO5 has been accepted as 'official' in the ROL/Castle agreement though now that Castle have looked to ROL for further development (eg ideas generated as Merlin project), and ROL have produced a 32 bit happy OS, the spotlight is bound to be in that area for further development.
The A9home is in the mainstream (and legal!) of RISC OS development.
And ROL is best left independent of all hardware developers where it can serve all development. It's how we got 32-bit happy RO4 and it's the only way RO can survive long term.