Apologies if this comment is similar to others as I note several people attempting to assist as I write …
If I was in your position I would ask to search the Minutes of the Meetings, particularly the General Meeting Minutes, to find out whether the correct procedures were followed which gave approval to the Lot owner to install solar panels on the common property roof.
Is the roof common property? How was this issue approved: Was a lease or license granted by Special Resolution? What were the terms and conditions that were imposed on the approval?
The Victorian Owners Corporations Act 2006 at Section 14 states:
Leasing or licensing of the common property
By special resolution, an owners corporation may lease or license the whole or any part of the common property to a lot owner or other person.
Answers to your questions:
Q1 – These types of approvals are on a first come first served basis. If the common property roof is “full” then unfortunately there is not much you can do about it.
Q2 – Why don’t you put a Motion on the Agenda that seeks to reserve a space for the common property panels should the Owners Corporation want to install panels at a future time? See Sections 52 and 53 of the Victorian Owners Corporations Act 2006 for the procedure for alterations and improvements to the Common Property by the Owners Corporation.
Q3 – Yes, most definitely. Owners should not only have been informed, they should have had an input into the approval process via a Special Resolution vote at a General Meeting.
The Act is here:
https://www.austlii.edu.au/au/legis/vic/consol_act/oca2006260/index.html
If you discover that something untoward has occurred then you have the option to use Section 152 of the Act, and the services of VCAT to resolve the issue.