The only information that you need to take to them is that under Fair Trading’s Common Property Memorandum, the owners corporation is responsible for the ceiling and common property walls (those adjacent to the external walls or other lots). Even if your OC hasn’t adopted this as a by-law, it’s still a very good indicator of the way the Tribunal will think, if it ever came to that.
You could then point them at Section 106.1 of the strata Act which requires them to fix common property and then at section 232.2 “Failure to exercise a function” which allows you to seek orders at the Tribunal if they refuse or ignore your request (which they have already done).
Also, I can’t find the reference to the legislation on this but, despite the principle that the paint inside your lot is your concern, I believe they are required to repaint the wall and ceiling after they have fixed it. It used to be part of the Common Property Memorandum but seems to have disappeared in later revisions.
I can’t find any reference anywhere to the 5mm rule and it sounds like one of those rules relating to other matters that strata managers latch on to, to keep the committee happy.