This link may help to answer your questions:
https://greenstrata.com.au/case/solar-power-common-property-personal-use-wallisview
As already noted by others on this forum it would not be fair to leave the one apartment, which is vacant, out of the process. They may opt not to participate but at least your endeavours to include them will be on the Minutes and should prevent any future negative blow back.
Best of luck with your solar plans.