tiamaria – in addition to the advice already provided, I should advise at the outset that (in NSW) all your Owners Corporation (O/C) needs to do in order to resolve its issues with those Owners who may in future wish to install air conditioning units is to Resolve (at a General Meeting) to create and register a Special By-Law in precisely the same way as the existing one covering the hotwater services (HWS) came about, including the Condition about the Owner/s being responsible for the on-going maintenance / repair / replacements of those.
As for the maintenance / repair / replacement of those air-conditioners that have already been installed, if those current Owners push the point that their air-conditioners are the O/C’s responsibility, then your O/C can Resolve (at a General Meeting) not to maintain those items provided that decision does not affect the safety of the building, the structure of the items, or create an eye-sore.
With regard to the HWS, individual Owners are required to cover their own costs of relocating their HWS to outside their Units and do the work in accordance with your Plan’s existing Special By-law, and even though it’s a good idea there’s no requirement (OH&S or otherwise) that I’m aware of that requires those internally located instantaneous (50L) systems to be so relocated. However, there is a Australian Standard (Plumbing Code) requiring those, and in fact all internally located HWS, to be installed on “Safe-Tray” so as to ensure that any leakage from the tank or the fittings is collected an drained through a common wall to outside the Unit or within the Unit to a nearby drain / floor waste.
You didn’t mention how an Owner’s desire to relocate a HWS to outside their Unit became linked to “advice” (from the O/C?) that they also install a vent to remove convey cooking fumes from that Unit; that association seems strange! There is a Building Code (in NSW) that for reasons of fire safety prohibits the fumes extracted by those types of vents from being ducted into the roof void, as an accumulation of cooking residues (fat & oil) can build-up in that area and become a fire accelerant.
I’m a bit your way concerning the potential for roof-mounted vent outlets to compromise the warranty of the new roof, after all tradespeople are very good at blaming each others’ work when there’s a problem (like a leak), so I’d suggest that the Owner concerned investigates a cook-top vent that is ducted outside through a common wall as opposed to through the roof, particularly as the longer vertical ducting of latter type typically becomes coated with residues and over time looses effectiveness; both options of course require your O/C’s prior consent.