From Strata Schemes Management Regulation 2010
24 Attendance and representation
(1) A mediation session must be attended by each party or by a legal representative, or other representative, having authority to settle the matter.
If they refuse to attend obtain this refusal in writing from the mediator and then proceed on to the Adjudication hearing and I would strongly suspect this would be taken into account by the Adjudicator as being unreasonable (so you’d get bronwie points).
The damage should be an insurance claim (unless they claim it was malicious) against Strata Insurance and the insurance firm would then take action against you to recover the cost BUT they would have to prove you were the cause (not just assume).
As for chooseing the 4K repair this was an OC decision and so you would have been able to vote on the motion that was put forward (should have been anyway). I don’t believe thay can refuse your vote just because of an unsubstantiated claim and you can get the adjudicator to consider this as well.
You can bring up multiple failures of the SM, EC and OC at the adjudication all for the same price $78.