The only way the Owners Corp can be compelled take over responsibility for air-con when it has been installed on common property is either
a) it was installed with permission but there was no agreement made that the owners would have responsibility for care and maintenance
b) it was installed without permission and then the property was subsequently sold to another owner.
Your by-law that everyone is responsible for their own A/C covers that.
If an owner wants to change common property, you can give them permission on condition that the changes they make are their responsibility, as is any deterioration of the integrity of the roof or walls as a result of the changes. If they refuse, then you refuse to give them permission.
The question of the warranty can only be answered by the installer. But if the owner wants a vent and you give them permission, they have to agree, as part of the requisite special resolution, to maintain and repait the seals around the vent and to make good any damage if they fail.
Y