First of all, it’s not the strata management firm’s responsibility, it’s the Owners Corporation (the strata manager is merely acting erroneously and over-zealously on their behalf).
Any excess payable on the insurance, and any other related loss, should come from the Owners Corp and they should get that back from the builder.
If the sale does fall through because of this incident, you need to get a notarised signed statement from the potential purchaser or their solicitor/conveyancer, stating this.
They don’t have to do this but they might do it as a favour (probably better if your solicitor talks to their solicitor). You then have something definite to use as ammunition in any action to recoup some of your loss.
That apart, any action you take would be a civil action through a local court. This is not a strata issue beyond establishing who should have done what and who is responsible. The answer to that question is that the builder should not have stuck a brick in the sewer pipe (which sounds suspiciously like a deliberate act of sabotage) and the Owners Corp are responsible for the consequences.
This problem is not likely to resolve itself. Talk to a lawyer, preferably one with strata experience, sooner rather than later.